This book is a treatise on Aristotelian assertoric syllogistic, which is currently of growing interest. Some centuries ago, it attracted the attention of the founders of modern logic, who approached it in several (semantical and syntactical) ways. Further approaches were introduced later on. In this book these approaches (with few exceptions) are discussed, developed and interrelated. Among other things, different facets of soundness, completeness, decidability, and independence for Aristotelian assertoric syllogistic are investigated. Specifically arithmetization (Leibniz), algebraization (Leibniz and Boole), and Venn models (Euler and Venn) are examined. The book is aimed at scholars in the fields of logic and history of logic.