This handbook, in five volumes, represents a complete course of liturgical studies which the Pontifical Liturgical Institute offers to professors and students of liturgy in universities and seminaries and to other persons engaged in liturgical ministry. It is offered as model, source, and reference. This second volume leads the reader to a deeper understanding of the liturgy by examining the basic concepts that belong to its definition. What concepts must one have in order to understand and explain the nature and purpose, the plan and actualization, and the relational character of the liturgy? This volume addresses these questions under three headings, which develop the foundational concepts of the liturgy, namely epistemology, celebration, and human sciences.