ROSAT observations, together with the continuosly growing database
of rotational velocities/periods for solar and late-type stars
in young clusters (in the 30-600 Myr age range), have permitted
to investigate in detail the rotation - X-ray activity relation.
It is now a well estabilished result that fast rotators exhibit a saturation of the level of their X-ray emission, i.e., the ratio of X-ray to bolometric luminosity is constant (L_X/L_bol ~ 10^-3) above a threshold velocity which depends on mass. On the other hand, the relationship for slow rotators is not so well defined.
In the present contribution I shall review the results on the saturation relation for rapid rotators and, using the most recent results on rotational periods for stars in the alpha Persei and Pleiades clusters, I shall address in more detail the issue of the dependence of coronal emission on rotation for slow rotators.