Without a more detailed description or photo, I am going to take a wild guess and say maybe you have aphids? Search for aphids images on the web. If they are indeed aphids, I generally kill them by hand since I only have a dozen tomato plants. You can also use diluted dish soap. Unless you have a huge amount of aphids, healthy tomato plants will usually outgrow them with little ill effect.

If the bugs are red rather than reddish-pinkish, very small rather than tiny, and seriously ugly, you have the voracious leaf-munching Colorado Potato Beetle larvae.
Google Images search for Colorado Potato Beetle larvae

I looked up the aphids and that is definately the bug. So a soap and water rinse should do the trick?

I had some woolly aphids on my plants and was able to get rid of most of them by blasting them off with the water hose (just don't blast so hard you damage the plants). The few that came back I just smushed with my fingers.

Here is a good publication if you like more details:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html

I have to disagree with everyone else for treatment. The soap is more likely to harm the plants than the aphids. No action is required. If you feel compelled to do something and must take action just knock them off with a hose.

Aphids usually come about from giving a plant too much nitrogen - lighten up on the fertilizer and they should go away. Also, if you are giving a tomato enough nitrogen to attract aphid swarms you will grow too much tomato leaf and not enough fruit. Getting you fertilizer right should be priority #1. Let the aphids go.

Aphids spread disease. You need to treat. Blast of water, soap sprays or neem oil are good treatments.

My Southern peas have aphids every year & I never add nitrogen to them. I have aphids on peas even when they do not bother any other garden plants.