This volume represents a collection of authoritative reviews by internationally recognized experts in the field of middle infrared (mid-IR) coherent sources and their applications. The compilation describes the fundamental principles and state-of-the-art of practical solid-state sources in the mid-IR regions of the spectrum covering the 2-50 m range. The described sources include, along with more traditional heterojunction semiconductor lasers, optical parametric oscillators, difference frequency generators, as well as the most recently developed broadband crystalline, quantum cascade and fiber lasers. The rapid advances in mid-IR science and technology have made these sources indispensable tools for a variety of applications in environmental monitoring and pollution control, agriculture and life sciences, and non-invasive disease diagnosis through breath analysis. Coherent mid-IR sources also offer important technologies for atmospheric chemistry, free-space communication, imaging, rapid detection of explosives, chemical and biological agents, narcotics, as well as for applications in air- and sea-borne security. In comparison to the previous volume on this subject in 2003, which focused mainly on the design and development of mid-IR sources, the emphasis in the present volume is shifted towards applications. The instructive nature of the book makes it an excellent text for practicing engineers, physicists and graduate students, who intend to develop or use mid-IR laser sources in various applications.