<p>In the last twenty years there has been a growing interest in middle infrared (mid-IR) laser crystals and their application to achieve mid-IR laser radiations which has benefited from the development of novel mid-infrared crystals and the improving quality of traditional mid-IR crystals. Moreover these works have promoted the development of related technical applications.</p><p>&nbsp;</p><p>This Special Issue of the journal <em>Crystals </em>focuses on the most recent advances in mid-IR laser crystals from materials to laser sources and applications. It aims to bring together the latest developments in novel mid-IR crystals improvements in the quality of mid-IR crystals mid-IR non-linear crystals and mid-IR lasers as well as the application of mid-IR technology in spectroscopy trace gas detection and remote sensing optical microscopy and biomedicine. Aspiring authors are encouraged to submit their latest original research as well as forward-looking review papers to this Special Issue.</p>