Louse eggs on the hair very close to the scalp are the primary sign of an active infestation. A female can lay up to 50-150 eggs in its life, but it can lay up to 100 eggs in 30 days. She also glues her eggs, sometimes called "nits", which look like tiny white beads, to hair shafts very close to the scalp (usually about a centimeter or two [half an inch to three-quarters of an inch] from the scalp). Eggs are very small, about the size of a period "." (full stop) in normal printing. Eggs may appear white, yellowish, brownish or greyish, but are almost always lighter colored. An egg normally undergoes a 7-9 day incubation before hatching as a baby nymph.

Classically, a louse egg does not become a "nit" until after it has completed its incubation stage, thus leaving a "nit." A "nit" is either the empty shell remaining after the nymph has departed or the dead egg that remains if incubation was not successful. Dead eggs will appear darker, or raisin-like, as they dry out. "Nits" of this sort are usually found over one centimeter (approximately one-half inch) or more away from the scalp and are not considered a sign of active infestation. Some people refer to lice eggs, live or dead, as "nits." In common usage, an egg that is still incubating may also be called a "nit."