In 1906, Santiago Ramón y Cajal was awarded the Nobel Prize in Physiology or Medicine in recognition of his work on the structure of the nervous system. At that time, almost all of Cajal's work was carried out using the Golgi method, a technique devised by the Italian scientist Camillo Golgi, with whom he shared this prize. Cajal introduced several modifications to the method developed by Golgi and, to avoid the problems encountered in staining myelinated neurons, part of his studies were carried out on embryos and very young animals (the "ontogenetic method"). In this way, Cajal begin to describe aspects of the development of the nervous system. Here, we review some of his wonderful discoveries (for example, the description of the axonal growth cone) from which he derived some of his main theories on the anatomy and physiology of the nervous system: the chemotactic hypothesis and the neuron doctrine.