I am very pleased to see that the vast majority of answers do not say that stretching is un-necessary. The first answer was very good, so I will only add this.
Yes, stretch prior to an activity to have the ROM that is required. You may need to do this by prolonging the warm up and using similar movements to those you will be performing. Or if you are already working out and find a joint/muscle ROM to be lacking or restrictive/uncomfortable, do a quick stretch for that joint/muscle. If the issue doesn’t resolve with the stretch, I would stop attempting that exercise/activity. You can do something else or you can end the training session and do a more complete stretch.
Now. Every workout should end with a solid and complete stretch of at least all of the joints/muscles involved in the focus of the workout. Time permitting, stretch all of the major muscles. It doesn’t have to be a long time, but each joint should be taken to the necessary ROM of the activity or to at least attempting to achieve an improved ROM for those with ROM deficits. And the stretches are held for 10 to 30 seconds at least 2 times, 3 times is better. If you don’t understand how to stretch, find some that does understand and pay them to teach you. If you want to come to Hawaii, I would be glad to teach you. My classes qualify for CEC credit with all of the major cert providers.
For those of you who feel stretching is un-necessary. What do you tell your clients with low back issues? Your clients with performance goals? What would you tell an athlete, dancer, weekend warrior? The stduies that most of the “no stretch” voices are quoting actually did not ever state that stretching is not needed. They said that stretching for improved ROM that is to affect the persons day to day ROM should be done at the end of the workout. This type of stretching prior to a workout reduced performance in the studies that were being done and/or reviewed. The vast bulk of research clearly demonstrates that stretching should be consider part of every workout/exercise program. And if you are a fitness professional and not teaching your clients how to stretch, you need to refer them to someone who can. Omitting this information is very unprofessional.
If you don’t believe me, find the entire study you think said otherwise and read the entire study or at least the actually summary. A sound bite is not solid information for forming an opinion.