My husband lived in these shirts during a trip to Costa Rica.  He's a runner and is very picky about the shirts he wears.  I insisted on these because of the SPF 50.  Once he started wearing these, he never wore anything else.  Freedom of movement, lightweight fabric, keeps you cool, dries quickly and SPF 50.

Ladies - buy these for yourselves, too.  I wear a women's M and wore the men's Small.  These (or the short sleeve version) were all I wore hiking, fishing, wandering around and swimming in the hot Costa Rican sun.  Only used SPF lotion/spray on exposed areas (hands, neck, head, face) and never got any color at all. (that's a good thing...)