The introduction to this volume is divided into two parts. The first part includes an overview of the state of the art on mental language as a key topic and tool in the philosophical analyses of the fourteenth century. We describe the current state of scholarship in five main areas: (1) the mental language hypothesis in general and in the work of William of Ockham in particular; (2) the comparison of Ockham to John Buridan (1295/1300–1358/61), another leading figure in the first half of fourteenth century, on mental language and related semantic issues; (3) situating Ockham within a broader context by examining themes in mental language in other philosophers both preceding and following him; (4) developments in Ockham's semantics and its connection to concept formation and cognitive psychology; (5) the relationship, if any, between mental language and nominalism. The second part of the introduction briefly describes the chapters of the present volume and explains their organization.