In Saint Diego of Alcalá, Francisco de Zurbarán presents a solemn depiction of the saint. He is shown in a simple robe, with a serene expression on his face. The color palette is dominated by earthy tones, with deep browns and soft whites that create a sense of calm.

The background is dark, emphasizing the figure of the saint and enhancing the spiritual atmosphere of the painting. The painting is executed in oil on canvas, a common medium for Zurbarán. The artist's technique is characterized by a smooth application of paint, which gives the surface a rich texture. Light plays a crucial role in this work, highlighting the contours of the saint's face and robe.

This careful manipulation of light and shadow is a hallmark of the Baroque style. Francisco de Zurbarán was a prominent Spanish painter known for his religious works. This particular painting, created in the 1630s, reflects the Counter-Reformation's emphasis on spirituality and devotion. An interesting fact about this work is that it was commissioned for a monastery, showcasing the importance of religious art during this period.

Zurbarán's ability to convey emotion through simplicity is what makes this painting significant.