Make sure you do a lot of research before you decide to get into breeding your geckos! You must be positive that you have a back-up plan in place in case you run out of room to keep all of your hatchlings. Gecko babies are not as easy to rehome as kittens or puppies - many animal shelters will not take them, and most pet stores are not allowed to buy stock outside of their predetermined sources. Read up on how to become a reptile seller or trader if that is your goal!
It is also a breeder's job to think about the ramifications of their breeding projects. If you own a sub-par or even defective gecko (perhaps your gecko's crests are very small, or it has an overbite or underbite), I recommend that you think twice about breeding that animal. It can still be a wonderful pet, but all pets do not need to be bred. The gene pool in captivity is regulated entirely by you and your fellow breeders - as long as we are "playing God" with their genetics, we should be mindful of possible outcomes. Would you want to buy a sub-par gecko for a future breeding project? If you wouldn't, why would anybody else? Make sure that any geckos you plan to breed are healthy and excellent representatives of their species.
I recommend that a female gecko weigh no less than 35 grams and be over 1 year old (for me, closer to 2 or 3; keep in mind however that geckos do grow at different rates depending on diet, temperatures, and genetics) before being introduced to a male for breeding. The male should be around the same size as the female, though in many cases the male being slightly smaller has not caused problems during breeding. The male does, however, need to have enough mass to be able to grip the female.
Sometimes a male and female might seem uninterested in each other, but often, mating takes place at night when you're sleeping. During copulation, a male will grab the female by her crests with his mouth and will make contact to her vent with one of his hemipenes. You may hear some "squeaking" sounds as they vocalize to each other. This act looks rough, and sometimes a gecko may receive bite-marks from mating, but they generally heal well if the enclosure is kept clean.
Don't be afraid if you see something red or pink hanging from the male's vent after copulation. This is the hemipene, and he will often lick himself until it fits back into the vent. If you see that your gecko is having trouble getting his hemipene back into his vent (a couple of hours after copulation), keep an eye on it and try to help keep it from drying out - some have recommended using sugar water or honey. You may need to call your exotics veterinarian if your gecko has issues re-inverting his parts.
If the mating was successful, your female will be gravid - not pregnant, as pregnancy indicates a live-bearing animal, not an egg-layer. About thirty days, give or take, after successful copulation, a female will look to lay two eggs in a soft substrate. If you are making use of a paper towel or similar substrate, you will want to provide the female with an egg-laying box. In my experience, it's a good idea to provide a lay box for any female over about 20 grams, as they can lay infertile eggs as well, and the box gives the gecko a comfortable enclosed space to dig and lay. I prefer to use a wide Gladware or Ziploc type plastic container with many ventilation holes and a little hatch for the female to crawl into. You can use many different kinds of substrate in this laybox, though I prefer a mix of very slightly moist sifted sphagnum peat moss and organic soil - if it still clumps together a lot, it is too wet - "just right" feels damp but still fluffy.
The female will often use her head and limbs to shovel as she creates a "nest" to lay in. She may spend all night, or even several nights, digging and burying herself in the substrate before she actually lays. Eventually she will bury the eggs in the substrate. Don't worry if you see your gecko fully or partially submerged in the soil - they can generally breathe quite well even if their heads are covered in the substrate. Pulling your female out if you see her head-first in the soil can disrupt her laying schedule.
I have read several sources advising to stay away from coco fiber/coir as a laybox and incubator substrate. At best, the material may stain the eggs, and it is thought that there may be a chance that the materials may leech into the eggs and harm them. I have not experimented with coir, so I cannot confirm or deny this claim, but I prefer to play it safe.
After eggs are laid, you may notice that your female has a bit of a "deflated" look anterior to the rear limbs, where she may have been a bit pear-shaped before, so that is something else to keep an eye out for if you're unsure if your gecko is gravid (carrying eggs).
A healthy female can lay a clutch of two eggs every thirty days or so. Viable eggs will generally look bright white all around - though if your eggs come out looking a little yellowish, it's recommended that you don't throw them out unless you're 100% sure they're infertile. Eggs can be incubated in a separate container; I prefer to use aquatic planting soil or APS as an incubation material. The APS should be rinsed thoroughly or even boiled ahead of time, and then allowed to sit in a sieve for a few minutes until it is cooled and no longer dripping. Aquatic planting soil can be found at many plant nurseries where koi pond supplies are carried. Use your finger to make indentations in the APS that will fit the eggs. The eggs can be set inside so they are only one-third to one-half submerged beneath the incubation material. An incubation container can have tiny holes punched into it to provide air for the eggs and eventual hatchlings, or, if you prefer to follow my methods, you can try it without the holes and just open the lid once a week for a few minutes to vent in new air. Just be wary of checking the container often once you know incubation time is nearing an end, so you can catch hatchlings and make sure they have enough air.
Gecko eggs are not like birds' eggs. They do not need to be rotated; they should be left with the same side facing up in their incubator through their entire incubation period. You can prevent shifting by making a small pencil mark gently on the top surface of the egg. In my experience, eggs will still hatch if you have an "accident" and they get bumped, so please do not panic if that happens; just keep in mind that in nature, the eggs would be buried under soil and left facing one way, so it may be developmentally helpful to keep from moving the eggs too much.
Incubation can go from around 65 days at room temperature, to all the way up to 120 days and beyond. If you keep your home on the cooler side, your eggs may take longer to hatch, but this is thought to be beneficial to the growing embryo, as it has time to absorb all of the egg's nutrients and may be stronger when it does finally hatch. In my experience, eggs incubated on the hotter side, at around 78 F+, seem to hatch out smaller with less-developed crests and tend to be a bit more flighty upon hatching. I find that about 70 to 72 F is a decent incubation temperature.
You can check to see if your eggs are viable by "candling" them. Take the egg into a dark room and hold it up to a very bright LED-type light. If you see a small red circle at the top of the egg, chances are, the egg is good. This red circle will develop quickly as the first week goes on, so if you didn't see anything the first day, try again in a few days. Keep in mind that the less you handle the eggs, the better, if for no other reason than to avoid accidents, so taking them out every day to candle them isn't a great idea.
Generally, the two eggs in a clutch may hatch within an hour to within a day of each other. However, it is not unheard of for a clutch to hatch around a week apart.
Please note, often first-time laying females have poorly calcified eggs, or they only lay one egg instead of two. As long as she is at an appropriate weight, is feeding well on a good diet, and shows no signs of illness, this should not be a huge concern. Usually a healthy gecko will get it "right" around her second or third clutch.
One problem to be aware of before attempting breeding is egg-binding, or your gecko becoming unable to pass eggs. If a female is undersized, does not find a suitable laying place, or there are other health issues present including a problem with calcium, she may retain the eggs. The eggs may continue to grow and possibly fuse together inside of the female's body, making it difficult or impossible for the eggs to be passed. If your breeding female's sides start looking bloated, or she becomes very lethargic and ceases or slows in food consumption, keep a close eye on her, but do not bother her overly much, as that can stress her out even more and prevent laying attempts. If you are expecting eggs and they are very long in coming, it may be worth your time to get your female checked at a reptile-knowledgeable veterinarian's office to find out if the eggs can be safely passed.
Once a gecko has developed fully inside the egg, it will break out using a pair of "egg teeth" developed solely for this purpose. Some have claimed that they are lost with the initial shed, but I have observed them (less noticeably) after this shed.
When hatchlings come out of the egg, they may stick their heads out and just rest for a while. Just let them come out on their own! They may take a few minutes or around an hour to fully emerge. Once they come out, be careful, as they can move very quickly. I keep two hatchlings of the same size in a small Kritter Keeper-type enclosure, with a very small bamboo branch and fake foliage for the first month. Their water and food dishes should be very shallow to prevent drowning, and their enclosure should be misted gently both in the morning and at night. While you can offer food right away, it has been my experience that most newly-hatched geckos do not show interest in eating for at least the first few days, so do not panic if you don't find food missing immediately. I personally do not recommend that you try to hand-feed the newly-hatched geckos, as it is incredibly easy for a small gecko to breathe in some of the food. Think about how things happen in nature - their parents would not bring them food. They will find the food on their own when they're ready! Other than that, their care is similar to an adult crested gecko.
A female gecko should be encouraged to take a break from breeding for a few consecutive months every year. One mating with a male can cause a female to lay fertile eggs for several months as their bodies can store sperm for some time, so if you want your female to stop laying, you may try a few different things. You may remove the male from her enclosure, attempt to bring the temperatures down to the high 60's, reduce the length of time your geckos receive light during the day, or a combination of these. The cooler weather and shorter photo-period will generally cause the gecko to stop laying eggs. Most breeders use the winter time when temperatures are naturally chillier to "cool" their geckos. During the cooling period, a female can rebuild stores of fat and calcium, and make her ready and healthy to go for the next season. Here in the northern part of the state of Illinois, my female geckos tend to stop laying in September, or October at the latest, without my interference, and generally pick up laying again in February or March of the following season. If you live in a warmer climate, you may need to take measures to make sure your females get adequate rest.
Crested gecko genetics are still being figured out; one may cross a dalmatian-spotted gecko with another dalmatian and their offspring might turn out to be a tiger-striped individual with few or no spots at all. It's exciting to see what your breeding pairs return!
Related Videos: Male Crested Gecko's Voice
Making A Crested Gecko Laybox
Crested Geckos Mating
Crested Geckos Not Quite Mating
Male Cleaning His Hemipene After Mating
Is Your Female Gravid?
Setting Up An Incubator
Incubation: Comparing Perlite and Aquatic Pond Soil
Female Crestie On Her Eggs Shortly After Laying
Crested Gecko Burying Eggs
Finding & Candling Eggs
What Happens To Infertile Eggs
Late-term Eggs Candled, Showing Movement
Crested Gecko Eggs Hatching