In the 15th century in Siena as elsewhere in Italy and Europe the successful painter had his own workshop. In his workshop he oversaw assistants and pupils that helped him finish the commissions the painter had received. It is indicative of Sano's success that over 270 of his works survive. It is one of the paradoxes about di Pietro that because he was so successful he is often overlooked. Many of his critics say that too many of his paintings resemble each other. It is important to remember that the painter produced what the customer asked for, not what he might paint for himself. The viewer must also take note of the fact that many of these works were not produced by Sano alone. In his "A History of Sienese Painting" George Edgall writes: