jderrick75's absolutely right about not stretching cold muscles, you'll get tears and strains if you do. if you're talking about stretching in relation to a work-out, then it's not strictly necessary to stretch beforehand (though a warm-up definitely is) although it doesn't hurt. but in order to avoid injury it's important to stretch after the work-out, this allows muscles to return to their normal length. if you're talking about stretching to improve flexibility, a warm-up is still necessary, and i'd advise doing a series of stretches for any given body part. i normally do groin, hamstrings, hips and repeat the body parts with different exercises. i've found dashama on youtube to be very informative in this area. i'd also highly recommend getting high as fuck beforehand, the combination is unreal (both in terms of results and the endorphin release).