 Quantum mechanics is usually taken to refer to the quantized version of a theory of classical mechanics, involving systems with a fixed, finite number of degrees of freedom. Classically, a field, such as, for example, an electromagnetic field, is a system endowed with infinitely many degrees of freedom. Quantization of a field theory gives rise to a quantum field theory. The chief philosophical issues raised by quantum mechanics remain when the transition is made to a quantum field theory; in addition, new interpretational issues arise. There are interesting differences, both technical and interpretational, between quantum mechanical theories and quantum field theories; for an overview, see the entries on  quantum field theory  and  quantum theory: von Neumann vs. Dirac.