In 1937, Pablo Picasso, aged fifty-six was considered world's foremost living painter. Due to his popularity the Government of the Spanish Republic commissioned Picasso an enormous mural painting of almost 8 meters width to be shown at Paris International world's Fair. Picasso's mind was blank for several months, but when Nazi air raiders destroyed the city of Gernika, his unchained anger led him to an intense and passionate creative process, which culminated in the most iconic of 20th century masterpieces. From to the 45 sketches that served to prepare the painting and the ten pictures his partner Dora Maar took during the creation of the painting we can deduce the process followed by Picasso during the 37 days of fury in which he painted Guernica. Note that along this paper we will use the official Basque name Gernika when referring to the town and the Spanish name Guernica when referring to the painting, as Picasso named it in 1937.