When the author began working on phosphors based on rare-earth elements, he lacked an introductory textbook that explained the fundamental chemistry, basic optical properties, and magnetic characteristics of lanthanide elements. This book provides a concise overview of the rare-earth elements and is divided into two parts. In the first part, the reader receives an overview of solid-state chemistry and fundamental physical properties of these elements. Key topics of the first part include the separation chemistry of lanthanides, their chemical behaviour and physical properties. Then relevant compound classes are illustrated, crystal structures are systematically explained. The second part focuses on the optical and magnetic properties on relevant examples, also discussing many applications. Students and researchers new to the topic of "Rare-Earth Elements" receive a comprehensive introduction to understand basic optical and magnetic properties and incentives for deeper studies.