This article describes the results of research whose main objective was to determine how important scientific corres- pondence and what role he played in the scientific work of Santiago Ramón y Cajal, considered the first neuroscientist of all time. Examined 360 letters exchanged between Cajal and other researchers of his time, we found that played an important role in the establishment of personal and scientific relations between researchers and supported, expanded and complemented the investigative work of Cajal, becoming the letters as part of his scientific Cajal’s Legacy consisting of about 4000 histological preparations, 2000 scientific drawings and over 300 publications