 Interference phenomena are a well-known and crucial aspect of  quantum mechanics,  famously exemplified by the two-slit experiment. There are many situations, however, in which interference effects are artificially or spontaneously suppressed. The theory of decoherence is precisely the study of such situations. It is is relevant (or is claimed to be relevant) to a variety of questions ranging from the measurement problem to the arrow of time, and in particular to the question of whether and how the ‘classical world’ may emerge from quantum mechanics. (See also the entry on  philosophical issues in quantum theory.)