Do you have to have a rest day?

Yes, they are necessary.

Working out stresses the body. That can be a good thing when you get proper rest. It is a bad thing when you don't.

Stress increases a hormone called cortisol. Resting allows your body to reset, heal and rebuild. If you don't get proper rest, the cortisol will become catabolic (eat muscle).

You should give your muscles 48 - 72 hours to recover from a hard workout. And yes, you should take at least one rest day each week.

Depends what you classify as a rest day. At present I'm running every day. I do though have a couple of days a week where I only run at a very easy pace for 20 to 30 minutes. I think the term 'rest day' doesn't help many people do a multiple of things throughout the week but would have what people see as a rest day but in fact be pretty active on that day but just doing something different and maybe less intense. The thing is to listen to your body and use a little common sense everyone is different.

I do at least 1 rest day a week. Mostly just to keep personal commitments, etc since it helps having a day/weekend where I don't need to budget out 2 hours for the gym on top of anything else I have going on.

There is really no reason why your body would operate on a 7 day cycle. I maybe take a rest day every 10 days (3 cheers for the metric system!) but more often I just try to take 3-5 days a month where I do active recovery (or cardio without resistance) knowing that at least 3-5 days a month I will also not have the energy or will to work out. I try not to schedule my rest days on a weekly calender because I know that sometimes I will feel ready to work out on a scheduled rest day but will really need the time off a few days later.

Take what your body needs... remember to explore the difference between when your body says "not today" and your mind says "I've had enough". Listen to your body but feel free to push back against your mind.

Your body gets the real benefit during recovery. When you exercise, you are deliberately tearing your muscle fiber. When it rebuilds, you become stronger. How is your body supposed to rebuild muscle fibers if you won't let it?

ETA: http://sportsmedicine.about.com/od/sampleworkouts/a/After-Exercise.htm

For strength training I know rest days are essential, because that's when the repairing of muscles happens. For cardio...I'm pretty sure (but not 100%) that you can go everyday if you want.

I agree with this. Muscles are torn down in the gym, and built when resting!

That said, I think rest days depend on a lot of things - age, sleep, nutrition, intensity and type of workouts etc. etc. You could probably do Zumba every day & be fine without rest days whereas you couldn't lift heavy every day of the week and recover 100% without rest days.

I only take them when my body screams out for them or I have over booked and can't make time for my workout. happens rarely, although with the added running I've been doing, my feet are calling the shots and i'm finding myself less motivated to hit the gym lately. Although yesterday I took my unmotivated butt there anyway and and super thankful i did. I feel much better today.. Running shoes will probably fix that problem..

I've been told at least 1 rest day a week for lifting. I alternate focus and type of lifting, but I still usually do skip one day a week. I don't't have a rest day from all of it though, usually. Good bad or in between. If my body is telling me to sleep (Like it did on Monday) instead.. then i will have a rest day.. otherwise I do what feels good. I also find that when I get good sleep I feel and do better. I aim for 7-8 hours a night. I have good and bad night.. but since I've been really trying to get to sleep it's been more good then bad.

I lift 5-6 days a week, run 5-6 days a week, and do class (zumba, combat, body pump, insanity, etc..) 5-6 days a week. I also play sports when the weather permits 1-2 days a week.

No rest day for the wicked or weary.

It depends on how you're training. If you're just going for walks or doing the elliptical or whatever you're proably fine without a rest day. You do need rest when weight training and should not train the same muscle or group of muscles on consecutive days...you need to either do a split or if you do full body just 3x weekly on non-consecutive days. Rest is where the magic happens.

Further, if you're training hard cardio and logging a lot of miles on a bike or running on your feet, etc a rest day is beneficial...those who work that hard and put a ton of mileage in week in and week out need to give their bodies some time to recover...

I always have at least one rest day...this doesn't mean I sit on my *kitten* and do nothing...I go for a walk or do a recreational ride with my kids vs an actual training ride. This time of year I generally take two rest days as my primary focus in the weight room and I just try to get in a couple of cycling days in to maintain some base but I'm not actively trying to up my mileage and I'm not training for a ride or anything.

When I get into the spring and start cycling a lot, I'll be putting in 100 miles + per week then I definitely make sure I have an actual and full rest day as well as short/less intense workout days disbursed my more intense days and longer mile days.

I do yoga on ''rest'' days. I consider yoga as rest but someone else might not.

I guess it depends.

For the sake of variety and differing perspectives, give this a read. The book is quite dense, but also fairly compelling.