Starting from basic concepts, this book provides a comprehensive review of cutting-edge experimental research in the field of infrared precision molecular spectroscopy and absolute frequency metrology. Ranging from the near-infrared to the THz region, state-of-the-art narrow- and broad-band coherent radiation sources with corresponding frequency stabilization methods are described. The most sophisticated high-sensitivity and high-resolution spectroscopic techniques are then discussed, along with an overview of the most recent record-breaking measurements. Relying on improved samples enabled by emerging technologies for producing cold stable molecules, in conjunction with superior frequency standards delivered by modern optical fiber networks, perspectives for further refining spectroscopic performance in terms of ultimate attainable precision are also considered. Finally, exciting new developments are discussed that encompass increasingly refined tests of fundamental physics, quantum metrology and sensing, quantum simulation and computation with ultracold molecules. The book is aimed at researchers passionate about infrared laser sources and molecular spectroscopy, both as a useful guide for the design of advanced experiments and as a text for advanced university courses.