Yellow leaves are one of the most common tomato problems home gardeners run into, and one of the most commonly misdiagnosed, because several completely different causes produce a similar-looking result. Working out which one you've got comes down to where on the plant the yellowing starts and what else comes with it.
Natural lower-leaf senescence is the most benign cause and the easiest to rule in. As a tomato plant matures and puts its energy into fruit, it's completely normal for the oldest leaves at the very bottom of the plant to yellow and eventually drop, one or two at a time, while the rest of the plant stays healthy and green. If it's just the lowest few leaves, the plant is otherwise vigorous, and it's already fruiting, this is usually nothing to worry about. Pinch them off and move on.
Natural lower-leaf senescence is the most benign cause and the easiest to rule in.
Overwatering and underwatering both cause yellowing, and both can look surprisingly similar at first glance, which is why checking the soil itself matters more than just looking at the leaves. Overwatered tomatoes tend to show yellowing that starts low and spreads upward across whole leaves, often with a slightly wilted or droopy look even though the soil is wet, because waterlogged roots can't actually take up water properly. Underwatered plants show drier, more brittle yellowing, often starting at the leaf edges and curling inward, with the soil visibly dry a few centimetres down. Tomatoes need moderate, consistent watering rather than either extreme: irregular watering is also the main cause of blossom end rot, where the base of the fruit goes dark and leathery.
Nitrogen deficiency shows a distinctive pattern: overall pale, yellow-green leaves, rather than a healthy dark green, starting with the older, lower leaves first. The yellowing tends to be uniform across the whole leaf rather than blotchy, and the plant's growth generally looks stunted or slow alongside it. A dose of a balanced or nitrogen-leaning feed usually greens the plant back up within one to two weeks if this is the cause.
Early blight is the disease to rule out, and it looks different from the causes above: yellowing leaves that also carry distinct brown or dark, concentric-ringed spots, usually starting on the older, lower leaves first and working upward. It's more common in humid or wet summers and where airflow around the plant is poor, so leaves that stay damp for long periods, from overhead watering or overcrowded plants, are the most exposed. Removing affected leaves promptly, avoiding overhead watering, and spacing plants so air can move through them all help slow it down once it starts.
If you're still not sure which of these you're looking at, check the soil moisture first, a finger a few centimetres down, then look at exactly which leaves are affected and whether there are spots as well as yellowing. That combination narrows it down to one or two likely causes almost every time.