This study analyses the scientific output of Pío del Río Hortega (1882-1945). A review was conducted of the content of his publications, articles he wrote in collaboration with other authors, and the profiles of the publications in which his work appeared. Analysing the scientific oeuvre of Del Río Hortega necessarily also requires analysis of the laboratory at the Residencia de Estudiantes. It was at this laboratory that he developed his histological school, beginning in October 1920, and where his scientific work gained clear international recognition. To that end, this article offers an overview of Del Río Hortega' s Spanish and foreign students at the laboratory of the Residencia de Estudiantes, shedding light on the relevance of the histological school he created. This study is based on a detailed review of various primary sources (Del Río Hortega' s doctoral thesis from the Universidad Central, annual reports of the Junta de Ampliación de Estudios [Board for Study Extensions], his autobiography, his correspondence, and the original documents of his publications), which are contrasted with the extensive literature on Del Río Hortega. Publications and their contents are contextualised with information on Del Río Hortega' s life, taking into account the immediate and lasting impact of his findings and the influence of the successive republication of English translations of his work (the majority of his work was originally printed in Spanish). The sheer volume of publications, and especially their quality and the originality of his findings, solidify Del Río Hortega' s status as one of the greatest ever microscopists in the medical sciences, becoming, after Cajal, the most distinguished member of what is universally recognised today as the Spanish histological school.