Contrary to popular belief, stretching before a workout does not appear to decrease the occurrence of injury. The risk of injury seems to be about equal for those who stretch and those who do not stretch before exercise. The warm-up, not stretching, seems to be the important deterrent for injury, performed before an exercise bout.
Although stretching does not seem to offer many short term benefits when performed before exercise, stretching does seem to offer other long term benefits. Improved flexibility may help prevent back and other orthopedic problems. Individuals with certain muscular imbalances or postural problems can benefits from stretching. Stretching can help maintain flexibility which may otherwise decline with age or inactivity due to an injury. Stretching may be more safely performed after dynamic exercise, when muscles are warm. Unless an activity requires extreme flexibility, stretching before is probably unnecessary. And even then, stretches should be performed before a warm up.
Stretching is not the same thing as warming up. Confusing stretching with warming up is an all-too-common mistake, so don't feel bad if you thought the two were one in the same. Warming up involves spending a few minutes doing lighter intensity activity that mimics your upcoming workout—walking before running, slow cycling before biking, light aerobics before a fitness class. That is a warm-up. It gives your body time to adjust to the higher demands of exercise so that your breathing rate, circulation and heart rate can all increase in order to supply your working muscles with the blood, nutrients and oxygen they need to keep things running smoothly. Warming up also helps lubricate your joints.
Stretching does not serve the same purposes and therefore does not pass for a warm-up. Stretching at the end of your workout, right after your cool down is best because your muscles and joints are much warmer and lubricated after a workout than they are before one , which means you'll get more out of your stretches at this time. And because your body is returning to a relaxed state, stretching after exercise is simply a feel-good way to end your workout.
Be smart, stay healthy.