Why have my tomato plants all have yellow leaves.?

it can be either too much water or not enough water. i have plants in full sun that are half yellow, and plants in almost full shade that are dark green. also, your pots may be too small (if they are potted). either way remove the yellow leaves if possible.

Yellow leaves on the bottom are common, remove and trash them. If all the leaves are yellow, then Linda is right.

Too much water, not enough sun.