Pío Baroja, considered one of the most prolific novelists of twentieth century Spanish literature, was born on December 28, 1872, to Serafin Baroja y Zarnoza and Carmen Nessi y Goni. Baroja had two brothers, Dario and Ricardo, and a sister, Carmen. The family eventually moved from San Sebastian, located on the northern Basque coast of Spain, to Madrid, where Baroja, at age fifteen, began to study medicine. He received his degree in 1893 after presenting his doctoral thesis on psycho-physical aspects of pain and moved to Cestona to work as a country doctor. He left the practice after only a year--tired, he said, of small-town life and the trivial rivalries of the profession--and moved back to Madrid to manage his aunt's bakery. In Madrid he began writing more seriously, contributing regularly to a journal called . When the bakery failed in 1902, Baroja devoted himself full-time to writing, often publishing more than two books a year until his death in 1956.

Baroja published (), a collection of short stories, and (), his first novel, in 1900. , (), and () form Baroja's first trilogy, (), completed in 1909. Baroja clustered many of his novels into cycles and series, the largest being the twenty-two volume (). Most consider (first published in 1900, and translated as in 1922) to be Baroja's greatest work. The semi-autobiographical bildungsroman details the life of Andres Hurtado, a young medical student who searches for meaning, confronts life's injustices, and ultimately commits suicide.

Like many Spanish novelists, Baroja wrote in a pessimistic and picaresque style. Most critics agree that Baroja belonged to the Generation of 1898, a loosely linked group of young writers preoccupied with Spain's social and political deterioration at the turn of the century. Much of Baroja's writing--characterized by its simplicity, directness, and use of colloquial language--focuses on the difficulties of modern city life for the Spanish underclass. Although Baroja practiced medicine for only a short time, his medical experiences left deep and abiding impressions. Baroja scholars estimate that 200 physicians appear in his collected works.

Baroja lived a fairly quiet and sedentary life in Spain, but would occasionally travel through Europe, especially to France, England, and Italy. During his travels he met other literary luminaries like Oscar Wilde and Spanish poet Antonio Machado, and he accumulated an impressive library of books about witchcraft and the occult. In July 1936, at the start of the Spanish Civil War, Baroja was imprisoned as "an enemy of tradition." Even though a member of the army recognized Baroja as a famous author and released him after a single night in jail, Baroja was outraged and moved to France. He didn't return until the end of the war in 1939.

Back in Madrid, Baroja published his memoirs. The six volume () appeared between 1944 and 1949. In his old age Baroja suffered from arteriosclerosis and memory loss. He died in Madrid on October 30, 1956, and family, friends, and writers attended his funeral. Ernest Hemingway, an admirer, was asked to be a pallbearer but declined, saying he felt unworthy of such an honor.