All mainstream database systems lack support for temporal metadata, and instead store only the most recent data. Consequently, they can only manage temporal data in an ad hoc manner -- oftentimes not effectively. Yet there are countless application domains that need temporal data. This book offers a much-needed solution: It includes a bitemporal relational data model, based on nested relations, as well as its algebra and calculus languages for expressing queries. The fundamental construct for representing temporal data is a bitemporal atom; a value, its validity period, and the time this data was recorded in the database. The concept of 'context' defines different views of bitemporal data. BtSQL, a simple bitemporal variation of the SQL, is introduced. The book also provides a prototype implementation, which demonstrates the feasibility of the proposed approach in an object-relational database system. Researchers and practitioners in database development will find this book useful. It can be used as a textbook in advanced database courses -- particularly in application domains where temporal metadata can greatly leverage the value of the data as it is stored and utilized.