Many fly species can be present in homes and other structures. Some flies, especially the smaller species (under 5 mm long), may reproduce indoors, laying eggs on unusual surfaces such as fungus growing on house plant roots, organic matter lining drains, overripe or rotting fruit, and other decaying, often hidden, organic matter. While most flies found indoors are a nuisance, they also can harbor bacteria and contaminate food or food preparation surfaces and should not be ignored. This publication describes four common flies about 1/8 inch (3 mm) long that can reproduce and develop indoors and discusses their biology and best management practices.