During the past several decades, tremendous progress has been made in terahertz (THz) science and technology. There is a continuing need to have terahertz waves ready for practical applications. Terahertz photonic and electronic devices are being readied to be employed in application systems such as communication links, satellite communications, radar, surveillance, hard soft material heating, biomedical treatment, and biomedical diagnostics. This book focuses on the advances in terahertz source technologies both from photonics and electronics (solid-state and vacuum-state) points of view. Written in a noncomplicated language, the book will be useful for a broad spectrum of readers, including advanced undergraduate- and graduate-level students in electronics and photonics, researchers in various disciplines in physics, chemistry, biology, astronomy, and electrical engineering, system engineers in various industrial sectors, general readers, and those who are interested in the interaction between electromagnetic waves and matters and in the effects of electromagnetic waves on matters. This book is organized into three distinct parts, each focusing on specific applications and breakthroughs of THz sources within their respective domains: Photonics, Solid-State Electronics, and Vacuum Electronics.