Most vegetables and herbs have mutually beneficial relationships with a group of “friends”. Their friends may protect them from pests and diseases or attract beneficial insects and pollinators. With the right companions, plants will be protected from problems and produce higher yields. A non-vegetable example is that chives planted near roses discourage aphids from damaging the roses.
Some plants have “foes” too. Foes can reduce yields and inhibit growth. For example, peas and beans don’t like to be near onions or garlic because they stunt their growth.
To have any meaningful impact, companions should be planted as neighbors in reasonably close proximity; they might share the same container, raised bed or raised row.
Friends and foes are listed for each vegetable and herb under.
Click here for a Vegetable and Herb Companion Chart which can also help you to choose beneficial companions for your vegetables and herbs.
GrowVeg.com can be helpful in laying out a garden using companion planting.
Visit the Vegtables Forum at GreatLakesGardeners.com to get answers to your growing vegetables questions. To ask a new question, Register if you haven't already done so(it's free and helps protect the forum from spam), click on Start New Topic, enter your question and click on Post New Topic.
You may also appreciate these books on growing vegetable gardens.