First, how are they turning yellow? Are the older leaves turning bright chrome yellow and dropping off, or are the new leaves turning chartreuse, with deeper green veins.
If the former, are just a few (less than 10%) turning? That could be just the older leaves dying of old age. More than that could be a watering problem--too much, too little too often, too irregular--or soil that is too heavy.
If the latter, what are you feeding it? Too little iron and/or too little acid could be the cause, especially in pots. In the ground, it might be the result of too little water, applied too often, causing a sodium buildup, and more alkaline soil.