Bernal Díaz del Castillo (1492 or 1493 - 1581) was a conquistador, who wrote an eyewitness account of the conquest of Mexico under Hernán Cortés. Born in Spain, he came from a family of little wealth and he himself had received only a minimal education. He sailed to Cuba in 1514 to make his fortune, but after two years found few opportunities there. Much of the native population of the island had already been killed by epidemics and forced labor, and in 1517 an expedition was sent to the smaller Caribbean islands to find alternative sources of labor. Díaz joined this group, under the command of Francisco Hernández de Córdoba. It was a difficult venture, and although they discovered the Yucatán coast, by the time the expedition returned to Cuba they were in disastrous shape.

Nevertheless, Díaz returned to the coast of Yucatán the following year, on an expedition led by Juan de Grijalva, with the intent of exploring the newly discovered lands. Upon returning to Cuba, he enlisted in a new expedition, this one led by Hernán Cortés. In this third effort, Díaz took part in one of the most successful military campaigns in history, which brought an end to the Aztec empire of Mesoamerica. During this campaign, Díaz spoke frequently with his companions in arms about their experiences, collecting them into a coherent narration. The book that resulted from this was The Conquest of New Spain. In it he describes many of the 119 battles in which he claims to have participated, culminating in the fall of the Aztec Empire in 1521.

As a reward for his service, Díaz was appointed governor of Santiago de los Caballeros, present-day Antigua Guatemala. He began writing his history in 1568, almost fifty years after the events described, in response to an alternative history written by Cortés's chaplain, who had not actually participated in the campaign. He called his book Verdadera Historia de la Conquista de Nueva España, i.e., "True History of the Conquest of New Spain," in response to the claims made in the earlier work.

Díaz died in 1581, without seeing his book published. A manuscript was found in a Madrid library in 1632 and finally published, providing an eye-witness account of the events, often told from the perspective of a common soldier. Today it is one of our most important sources for understanding the campaign that led to the collapse of the Aztec Empire and the Spanish conquest of Mexico.