I ordered 4 larges and 2 mediums for my relay teams (high school).  Only one boy (no girls) were able to wear the medium.  The larges worked well.  They are very tight and I would only recommend for sprint events.  The common complaint from my swimmers were that they were very hot and they waited til the last minute to put them on.

To Joseph S.: These caps are supposed to be small! Being a wearer of one, I find that, although tight, these caps are very effective!

The cap is well designed.  The issue is size. I ordered size medium because: (1) I do not have a large head,  and (2) my head is clean shaven.  I can get the cap on, but it is very, very tight.

I did look at the sizing information on the Speedo website before selecting size medium. The information is not useful.

This is a great product. The only concern is that it comes in Medium and Large but there is little to suggest which size will be appropriate and unless you previously owned one, you would not know which size to purchase. The Medium is really quite small.

It fits great.  My son uses it during swim meets.  It stays secure and fits snug.

I  am an adult male.  I have a small head but ordered the medium size.   It is almost impossible to put this on my head.