Should I Run Outside or on a Treadmill?

In my opinion, (which I agree with 100%), if you want to go for run, by all means do it outside.  We spend enough of our lives inside - at work, at home, in the car, etc...  Fresh air and sunlight are essential for good health.  Personally, I find treadmills somewhat boring since I feel like I never get anywhere.  Also, if you stumble outside, you can probably catch your balance and keep going.  If you stumble on a treadmill, it can be disastrous!  - OUCH!

However, there are times & reasons when a treadmill workout is actually a good way to go.

1. When the weather is really bad outside - downpours, extreme cold, extreme heat, high winds, etc...then run a mile or two on the treadmill.  It makes no sense to go outside in the extremes if you don't have to.  This is from both a  health and safety approach.  In fact, if you're the type of runner who would even consider training in those elements, you may need an easier day anyway.
2. If you are just getting back into running, a treadmill can be a great way to learn what a given pace feels like.  I prefer to do this on a track, but the local track has been down for repairs.  I've found the treadmill a great way to do my "track workouts". 
3. While you are on vacation, a treadmill in the hotel will be there when you can run.  It won't matter if it's dark, or an unfamiliar city. 
4. If you are somewhere flat, and want to train for hills, you can set the incline to mimic a hilly course.
5. Treadmills tell you all sorts of good stats: Distance, time, pace, calories burned, etc...
6. Missed your run today?  Hop on the treadmill while watching T.V. or listening to your music - sometimes this is a good enough reason since I don't recommend wearing headphones while running, but plenty of times I wish I had some motivating music to keep me going.

I'm sure other runners could rattle off numerous other benefits, but these are the ones that impress me.  I would offer a couple of suggestions though:

•  Start off slow and increase your pace gradually if you are not used to a treadmill.  Some models accelerate rapidly toward the pace you set it at.  This can catch some people off guard.  (I speak from experience.)
• If you can set the incline, set it to at least a 1% grade.  This will more closely mimic outside "flat" terrain.

Hopefully that helps put into perspective the pros of using a treadmill when running outside just isn't an option for you.

Happy Trails!