A composting toilet is any system that converts human waste into an organic compost and usable soil, through the natural breakdown of organic matter into its essential minerals. Aerobic microbes do this in the presence of moisture and air, by oxidizing the carbon in the organic material to carbon dioxide gas, and converting hydrogen atoms to water vapour.

"Self-contained" composting toilets complete the composting "in-situ,", while "central unit" ones flush waste to a remote composting unit below the toilet. Vacuum-flush systems can flush horizontally or upward.

Composting toilets can be installed anywhere, such as a cabin, cottage, bunkie, yurt, RV, pool cabana, boat, shed, barn, or home.

Some composting toilets use electricity, and some electrical systems use fans to exhaust air and increase microbial activity. Others require the user to simply rotate a drum within the composting toilet to allow for an aerobic breakdown of waste.

Some composting toilets have a large compartment below the toilet. Others are little larger than a traditional toilet.

All composting toilets eventually need some end-product removal. A full-size composting toilet does not need to have solids removed for several decades if the active tank volume is at least three times the yearly addition. This is because the waste dramatically decreases in volume: after around 5 years only 1-2% of the original volume remains. It is then a mineralized soil, which will not decompose any further. Other smaller systems may need to remove solids several times a year.