The book reviews our present knowledge of collision-induced absorption of infrared radiation in dense gases. The book starts with a recapitulation of essential background information. Experimental results for the absorption spectra are next discussed. Then the causes and properties of dipole moments induced by molecular interactions are reviewed. Two following chapters present the theory of collision-induced absorption in monatomic gas mixtures and in molecular gases and mixtures. The final chapter discusses related phenomena and important applications in astrophysics. The book is a practical guide for the spectroscopic dealing with dense, neutral fluids.