Tomato Leaf Curl

While leaf curling isn't deadly to your plant, it is a sign that something should probably be corrected. It is a physiological response to stress of some sort.

Overwatering. An often overlooked reason. Roots can be damaged by too much water.

Underwatering. The leaves are trying to prevent further water loss, because the plant is dehydrated!

Extreme heat. Tomatoes aren't as sun-loving as you may think. Once temperatures rise over 90°F, the plant may start dropping blossoms and cease growth. Higher than that and for long enough, and the plant will be very stressed to the point of burning or death.

Too much nitrogen. If you've applied a fertilizer with too much nitrogen during the fruiting stage of the plant's growth, the leaves will curl.

Excessive pruning. This can stress the plant, causing leaves to curl.

Herbicide. Nearby herbicides can stress the plant. Very rare.

Virus. Very rarely, it may be the result of a virus. This is incurable. Remove and destroy the plant. Do not compost it.