Next: X--ray Activity and PMS stars . ... This has made the need to study the evolution of the rotation periods of stars, and hence the angular momentum, very clear. ... In the case of low mass pre--main sequence (PMS) stars, we seek to learn whether disks alone determine the angular momentum distribution when stars arrive on the main sequence. ... If one assumes that angular momentum is conserved from the natal cloud, then rotation periods exceeding breakup velocity are implied for PMS stars. ...