Coherent population trapping (CPT) is a technique in ultrahigh-resolution spectroscopy, developed in recent years. It relies on the nonlinear coherent interaction between an atomic system and incident electromagnetic radiation and is followed by an analysis of the fine structure of the response to this interaction, which contains information about the spectral characteristics of the quantum system. The basic physics of CPT are reviewed, and the effects of level relaxation and structure, and of external factors, on CPT dynamics and characteristics are examined. Experimental results confirming the validity of theoretical ideas on CPT are presented. Possible applications of CPT to frequency stabilization, laser cooling of atoms, and so on are discussed.