People lose hair as they get older or because of things like stress or medical problems. The medical word for hair loss is alopecia (say "al-uh-PEE-shuh").
The most common type of hair loss is genetic. It's called male-pattern or female-pattern hair loss. Men generally get bald spots on the forehead area or on the top of the head. In women, the hair on the top of the head thins out.
Other types of hair loss can cause hair thinning or clumps of hair to fall out.