Most tomato gardeners have seen the symptoms: lower leaves turn bright yellow then drop off the plant. As the season progresses most leaves may drop off the tomato vines, leading to sunscald of fruit if any form.
The disease is early blight, Alternaria solani. The fungus is present in most soils and can not be eliminated. The best practice to limit the disease occurrence is to mulch under the plant IMMEDIATELY after planting. In this way infected soil can not splash onto lower leaves.
If the disease has just gotten started, spraying upper leaves with a fungicide labeled for garden use (Daconil, etc) may be helpful.
This factsheet from Mississippi State University (never to be confused with the University of Mississippi!) has some good tips: