I don't have time to go to the gym. Will doing 100 pushups a day keep me in shape?

No.

It’s a start – but you need cardio, and at least abs and legs too – pushups are almost only arms. Besides, doing them incorrectly will be a serious dis-service to yourself.

Get a routine from a pro.

Pushups are ok, but in excess they can cause joint problems. Nearly all your weight repeatedly being on your wrists and shoulders eventually puts a strain on those joints. If you want to do pushups I’d suggest doing it as part of a more diverse routine.

As far as cardio.. you don’t necessarily have to run run… you could do something like play basketball or racquetball or soccer.. all 3 are vastly more entertaining than just running alone.

Get a jump rope and some weights.

I like pushups, too, for strength training. But they not a subsitute for longer periods of aerobic exercise. The bars pretty low – walking counts. Swimming, games, jump rope, weights – as suggested above, choices are many.

This is probably more what your doctor would have in mind:
http://www.mayoclinic.com/health/exercise/AN01713
Exercise: How much do I need every day?
How much should the average adult exercise every day?
Answer
from Edward R. Laskowski, M.D.

For most healthy adults, the Department of Health and Human Services recommends:
* At least two hours and 30 minutes a week of moderate aerobic activity (think brisk walking or swimming) or one hour and 15 minutes a week of vigorous aerobic activity (such as running) — preferably spread throughout the week
* Strength training exercises at least twice a week
As a general goal, include at least 30 minutes of physical activity in your daily routine. If you want to lose weight or meet specific fitness goals, you may need to increase your activity even more. If you can’t set aside time for a longer workout, try 10-minute chunks of activity throughout the day. Remember, the more active you are, the greater the benefits.

Anything is better than nothing. Just keep moving so you don’t get that old man shuffle when you’re 70.

If you can get to the point where you can do 5 sets of 100 push-ups, then you would be in shape…

it’s definitely better than nothing because you are working your chest muscles which is one of the largest in your upper body, and of course it does hit other areas but doesn’t target. Its also great for your core. But in terms of staying in shape you do need more than just that. It also really depends what you mean when you say in shape (strength or resistance training, endurance?) you should of course do some kind of cardio as well for at least 30 min 3 times a week. Cardio also doesn’t have to be rigorous, but as long as you do something that raises your heart rate a little bit more than the usual will do. work on your core a lot, it will help you over all with everything because it’s like the center of your body. all kinds of ab work s great, all kinds of planks are the best. also, if you can, try to work out in the mornings, it helps kick start your metabolism so you burn more calories throughout the day doing the same exact things. remember to stretch before/after a workout, its really important

this article on stretching is really interesting
http://www.askmen.com/sports/bodybuilding/57_fitness_tip.html

There is not any ONE exercise that will do it, you need a variety.

@DrBill
I think weighted squats come pretty close. But then again those leave out most of the upper body.
For a minimalist strength work-out schedule that still works the entire body, I’d recommend squats, push-ups and pull-ups. You could do squats on Mondays and Thursdays, push-ups on Tuesdays and Fridays, pull-ups on Wednesdays and Saturdays and take the Sunday off.

As for cardio, or indeed for all exercise, the general rule of thumb is that settling for less will achieve less. It’s up to you to decide how awesome you want to become. :)
But I would say anything that gets your heart rate up would in principle suffice, if you only want to stay in your present shape.

It will work your upper body.

If you don’t like running (I hate it personally) what about biking?

I used to say I didn’t have enough time to go to the gym either. but really all you need is a couple times a week. I go for about 45 minutes a day… right after work. but you cant spare 30 minutes twice a week?

I wish I were that busy. :)

@mowens
You don’t even need to go to a gym, there’s enough you can do at home or wherever.

I’m doing an internship right now. I’ve already found two tables at my “work place” where I can do dips during breaks, a floor where I can do normal and handstand push-ups and planks, and an overhanging balustrade where I can do pull-ups. You can do most of this anywhere, really.

you want to be hard in the middle and squishy on the edges…?

As others have said, doing push-ups will work your upper body, and doing only push-ups may leave you open to injuries from repeated use. If you must, see if you can find a copy of the Royal Canadian Air Force Exercise Plans for Physical Fitness or something similar and work up a set of exercises to maintain strength in the rest of your body. Then find several cardio activities you enjoy, such as walking a dog, playing a pick-up game of basketball or soccer with friends, riding a bike, going for a swim, playing ping-pong, or whatever, and do at least one three times a week.