Head lice are pretty hard to spot. Besides the fact they are pretty small, you have tens of thousands of hair strands on your head that are doing a good job of covering them.
Lice eggs (called nits) are so small they look just like tiny specks. You'll find them attached to individual hair shafts about a quarter inch (~0.5 cm) off the scalp. You can see them with the naked eye, but unless you've seen them before, you may mistake them for dandruff.
An adult louse is about the size of a sesame seed. Adult lice also like to crawl, so they are the easiest to spot.
After hatching, a louse nymph will take 9-12 days to grow into a mature adult. During this time its size can range anywhere from a tiny speck to a sesame seed. If you pick up a dime and look at FDR's ear (see our lice pictures), a nymph would probably fit inside his ear.
Think you might be infested? How you know if you have head lice.