This monograph introduces to the reader the idea of interference alignment, traces its origins, reviews a variety of interference alignment schemes, summarizes the diverse settings where the idea of interference alignment is applicable and highlights the common principles that cut across these diverse applications. The focus is on theoretical aspects.1 To keep the discussion general as well as compact we do not specify if the symbols are from a finite field or if they are real/complex, as the same ideas apply in both cases. In the latter case, however, it is worth mentioning that as usual there are additive noise terms and power constraints present, that are not shown in these equations, but without which the communication problem would be trivial, i.e., the infinite resolution of a real/complex number will allow infinite information transfer in any signal dimension.