Companion planting is the practice of planting symbiotic plants (those that benefit each other) together to foster symbiotic relationships between the plants. Companion plants can help spur each other's growth, deter pest, attract beneficial insects and organisms, and increase yields. A very common example is the three sister guild: corn, beans, and pumpkin. The corn plants act as climbing aid for the beans, the beans provide the soil with nitrogen, while the pumpkin acts as a living ground-cover keeping weeds under control and maintaining soil moisture. You can use the below companion planting list to find out which plants go well together and which don't.
most garden crops
Basil enhances the flavour and growth of most garden crops. Basil also repels mosquitoes.
beets, cabbage, carrots, catnip, cauliflower, corn, cucumbers, marigolds, potatoes, savory, strawberries
fennel, garlic, leeks, onions, shallots
Potatoes and marigolds repel Mexican bean beetles. Catnip repels flea beetles.
corn, marigolds, potatoes, radishes
beets, garlic, kohlrabi, leeks, onions, shallots
same as for bush beans.
broccoli, brussels sprouts, bush beans, cabbage, cauliflower, chard, kohlrabi, onions
charlock, field mustard, pole beans
squash, strawberries, tomatoes
Repels tomato worms. Improves flavor and growth of companions.
beets, buckwheat, calendula, carrots, chamomile, dill, hyssop, marigolds, mints, nasturtiums, onions, rosemary, sage, thyme, wormwood.
Marigolds repel cabbage moths. Nasturtiums repel aphids.
broccoli, brussels sprouts, celery, chard, spinach, tomatoes.
tomatoes and celery repel cabbage worms.
cabbage, chives, early potatoes, leeks, lettuce, onions, peas, radishes, rosemary, sage, salsify, wormwood.
Onions, leeks, and wormwood repel carrot flies.
apples, berries, carrots, grapes, peas, roses, tomatoes.
Improves flavor and growth of companions. Deters aphids and Japanese beetles.
beans, cucumbers, early potatoes, melons, peas, pumpkins, soybeans, squash.
Soybeans deter chinch bugs.
beans, cabbage, corn, early potatoes, radishes, sunflowers.
Radishes deter cucumber beetles. Cucumbers encourage blight in late potatoes.
broccoli, brussels sprouts, cabbage, cauliflower, cucumber, lettuce, onions
Improves flavor and growth of cabbage family plants.
green beans, peppers, potatoes, tomatoes
all garden crops
improves vegetables' flavor and growth.
most garden crops, especially lettuce, cabbage, cane fruits, fruit trees, roses, tomatoes.
legumes, beans, peas, potatoes.
garlic's natural fungicidal and antifungal properties help deterring aphids from lettuce and many common pests from cabbage.