What kind of yellowing, Malcom? On older plants, it is normal for the oldest leaves to turn yellow, but that is rarely more than 10% of the foliage. More extensive yellowing is usually a sign of nutrient deficiencies, or watering problems--the exact appearance of the foliage is a clue to the exact problem.

Use proper tomato food to feed the plants if you're not already - that contains the right amount of potassium and magnesium. Otherwise, as Tugbrethil says, need a photo or more info.