I have bought these in the past and follow the size chart found on the manufacturer's website, CW-X.  There is no doubt for me that the support offered by these tights, especially around the knees, helps me go farther without soreness in the joints.  I use them for biking, walking etc.  However, the tights I ordered from Amazon were a bit too small, despite ordering my normal size.  Bad luck with bad lot?  Don't know.  Will try again when I need a new pair.