The Index of Jewish and Crypto-Jewish Surnames is, to date, the most complete and reliable compilation of Sephardic surnames available. It brings together more than 30,000 names from Inquisition trials, migration records, civil and parish registers, as well as genealogical archives from several continents. Written in clear and accessible Spanish, this research book is not just a list: it is a reference guide for anyone interested in their Jewish roots, and especially for the descendants of Jews from the Iberian Peninsula. Throughout its chapters, the author explains in detail a rigorous methodology for assessing the probability of Jewish origin for each surname, combining a system of degrees of certainty (from A to C) with criteria of historical proximity, territory, frequency in documented cases, and family continuity. The reader will also find reflections on Jewish identity, the role of genealogy and DNA, as well as a clear explanation of the legal frameworks in Spain and Portugal and the role played by religious communities and institutions in recognizing Jewish ancestry. In addition to the theoretical aspects, the book offers practical tools for beginning or deepening family research: how to gather documents, correctly read old records, interpret witnesses and evidence, trace a lineage through the centuries, and avoid common mistakes when working with online family trees. The final section presents a comprehensive alphabetical index from A to Z, with thousands of surnames linked to Jewish or crypto-Jewish families according to the sources consulted. It also includes a list of the main Batei Din (Orthodox rabbinical courts) recognized by the State of Israel for the purposes of conversion, return, and citizenship processes, making this work a bridge between history, family memory, and current avenues of legal recognition. Designed for both professional researchers and readers without prior training, this index can serve as a starting point for personal study, as supporting documentation in legal or religious proceedings, or simply as a window into the hidden history of Hispanic surnames. Anyone who has ever wondered whether a Jewish branch runs through their family tree will find in these pages a map, a method, and numerous concrete clues to continue tracing their ancestors' path.