Reptiles are ectothermic, which means they don't generate enough heat to keep themseleves at a stable temperature. In order for them to stay active, they need to find an external source of heat. In the wild this is usually the Sun. Inside in a captive situation, we need to provide heat for our animals.

Thermostats
Thermostats are essential for controlling the temperature of an enclosure. A thermostat simply turns the heat source on when the ambient temperature drops below a certain point, and off when the ambient temperature rises above a certain point. This means that you can set the temperature of the enclosure to whatever best suits the animals you are keeping.
There are two main types of thermostats: those that have a remote temperature probe, and those that don't. A thermostat with a temperature probe can be wired anywhere within the enclosure, and the temperature probe is then placed at the point where the keeper requires the temperature to be regulated. This type of thermostat is usually more expensive but also more accurate, and it allows the thermostat to be places outside the enclosure. A thermostat without a probe (such as the commonly used IMIT) must be placed inside the enclosure at the point where the keeper requires temperature regulation - typically on the back wall 2/3 of the way up the height of wall, and 1/3 of the way along the length of the wall. These are less expensive, less accurate, but tend to last and work forever!
It is important to realise that the thermostat only regulates the temperature at the point of the probe (probed thermostat) or at the point of the thermostat (non-probed thermostat). This means that the temperature at other points in the cage could be above or below that of the location of the thermostat. This is both a good and a bad thing. In most enclosures it is best if there is a temperature gradient (warm to cool) across the enclosure, so that the animal can choose the temperature that best suits it (like putting on the right amount of clothes in the morning), and regulate its temperature by moving from warm spots to cooler spots. The down side is that the temperature at other places in the enclosure could rise or fall substantially without you knowing. However, by using a thermometer, you can monitor the temperature around the enclosure.

Thermometers
A thermometer is used to monitor the temperature of the enclosure. Any type of thermometer can be used, but it is important that it is accurate and safe. An alcohol filled (red inside) medical or scientific thermometer is quite good, as are those available from pet shops and reptile supply companies. It is best if you have at least two thermometers for each enclosure, so you can measure the temperature in different places within the enclosure. It is important to understand that the temperature within the enclosure will fluctuate over a 24 hour period, so a max/min thermometer is probably best, so you can see the current temperature, as well as the maximum and minimum reached. You can obtain digital thermometers (INSIDE/OUT) quite cheaply that read the temperature at the unit, and at a probe on the end of a 3 meter wire. These also record the max/min temperature reached. This type of thermometer is quite suitable, especially when mounted inside the enclosure near the thermostat, with the probe at the other end of the enclosure, giving temperature readings at each end of the thermal gradient. Thermometers SHOULD NOT be placed on a radiant heat source, or directly under a light/heat source. In this case, the temperature recorded will be much higher than the ambient (air) temperature to which your animal is exposed.

Placement of heat sources, thermostats and thermometers.
Heat sources should be placed within an enclosure to create a temperature gradient. This is so the animal has a range of temperatures to choose from, and can regulate its temperature of its own accord. Typically, the primary heat source should be placed approximately 1/3 of the way along the length of an enclosure, in the middle of the roof focused towards the floor in the case of a light, and in the middle of the floor in the case of a radiant heat source. At this position, one end of the enclosure will be the 'warm' end, whilst the other end will be the 'cool' end.
Thermostats should be placed so that they regulate the temperature around the position of the heat source, but never on or directly adjacent to the heat source. If the heat source is placed in the middle of the enclosure, the thermostat should be mounted at the back or side, 2/3 up the height of the wall, so that the ambient (air) temperature is being regulated, not the heat source's temperature.
Thermometers should be placed either end of the enclosure, approximately 1/5 of the length from the ends, and 2/3 up the height of the wall. This ensures that the ambient (air) temperature is being recorded at each end of the temperature gradient.

Hot rocks/Hot Boxes/Hot Logs
Hot rocks and hot boxes are a source of ground radiant heating for the enclosure. In most cases, a hot rock SHOULD NOT be used as the primary heat source. Also, hot rocks are notorious for causing burns on animals, as they overheat when the animal is lying on them. For this reason, it is important to have any hot rocks on a sensitive thermostat, or have a dimmer switch attached to ensure they are kept at reasonable temperatures.
A hot box is a wooden box that has a standard incandescent bulb (or two) inside. Because the lights produce heat, the box heats up, hence getting the name hot box! The bulbs should be low wattage (25 watts is standard), to ensure that not too much heat is produced. Also, the box MUST have ample air space around the bulbs to ensure they do not become a fire risk! It is normal to have the sides and bottom of the box made from thick wood (17 - 19mm) and the top slightly thinner (12 - 15mm), so the box is stable, and the heat escapes through the surface is the top. Some small holes drilled in the sides for ventilation will help decrease the fire risk, but MUST be smaller than the animals you will be housing (and their food source - you don't want hundreds of crickets living in the box!).
A hot log is very similar to a hot box, in which a light bulb or two is placed inside a hollow log. The fire risk in this situation is increased, as often hollow logs are still rotting/dropping rotten wood, and this can land on the bulb and ignite.
If you use any of these heat sources, they SHOULD NOT be placed underneath another heat source such as a lamp, as they will absorb the heat from the lamp as well as producing their own heat, and the surface can get VERY hot.
Heat mats

When heat pads go bad.
This heat pad went haywire and melted, burning the enclosure and the shelf on which it was sitting.

Infrared light (or using lights as a heat source)
There are three main types of light that can be used as a heat source:

1. a standard incandescent bulb, either coloured (party) or standard (household) globes
2. an infrared heat spotlight
3. a ceramic heat globe

Fluorescent and halogen bulbs themselves emit very little heat, but the transformer (or ballast) can emit quite a lot of heat. When setting up the enclosure, it may pay to consider placing the transformer/ballast in a position that maximises the use of this heat. This shouldn't be considered as a heat source, rather as a heat supplement. Also, please remember not to allow the animal access to the transformer/ballast or any associated wiring!
Standard incandescent bulbs produce a fair amount of heat - just put your hand near a lamp to see how much! A standard bulb can be used during normal daylight hours, but it has been suggested that 'white' light during the night will disturb an animal's "biorhythm", or internal clock. If you need to heat the enclosure at night, you can still use an incandescent bulb, it just needs to be a coloured party globe. Red and blue are the best colours, as reptiles' eyes do not respond to these wavelengths of visible light effectively, and so to them the enclosure will seem 'dark'. It is important that the globes are coloured all the way down the neck, otherwise the globes will still emit white visible light. The beauty of coloured globes is that it also allows you to observe your animals' nocturnal activities, albeit in a strange light!
An infrared heat spotlight is great as a day and night heat source. Because the light emitted is infrared, the animals can't see it, but it does heat the enclosure. These lights typically emit a large amount of heat, so require a ceramic fitting to prevent the heat from damaging the enclosure. Red light is also emitted by these bulbs, so these also can be used for night observation of your animals.
Ceramic heat globes consist of an element encased in ceramics. These are true infrared emitters, emitting only heat, and no visible light. These are great for heating enclosures day and night, and also in cooler climates, as they produce quite a lot of heat. These also require a ceramic globe holder, as they will melt plastic fittings. The downside to these bulbs is that they are fairly expensive, and emit no light so you can't see your animals at night, or easily tell if the light is working.
All heat bulbs should be attached to a thermostat to prevent over heating (i.e. cooking) of you animals. Also, because they are emitting HEAT, animals can burn themselves if they touch/wrap around the bulb. (If you don' t believe this, try touching a 100 watt bulb after it's been on for just a couple of minutes! Not recommended!). For this reason, all heat bulbs should have a mesh screen covering which prevents the animal from contact with the globe. Make sure the mesh is not too close to the bulb, otherwise the mesh itself will also heat up, and could cause burns.

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