Abstract The Historical Atlas of Spanish (AHE) is a free-access tool on the Internet designed to advance the knowledge of dialectal variation in the history of Spanish. The Atlas consists of a database of structural features (with preferential attention to morphosyntax) extracted from dialectal textual corpora. For each of the structures studied, the results are presented in the form of a brief state of the art illustrated with maps and graphs. The Atlas aims to contribute to the study of linguistic variation and change from a dialect perspective and extract significant generalizations about the diffusion of linguistic changes in the physical space. In this article we present the main methodological issues that have been considered in the process of creating the Historical Atlas of Spanish: contents and structure, data sources, techniques for data extraction and analysis, and the computer application for creating the maps and visualizing the results.