 In one interpretation, holism is a methodological thesis  (Section 2),  to the effect that the best way to study the behavior of a complex system is to treat it as a whole, and not merely to analyze the structure and behavior of its component parts. Alternatively, holism may be taken as a metaphysical thesis  (Section 3):  There are some wholes whose natures are simply not determined by the nature of their parts. Methodological holism stands opposed to methodological reductionism, in physics as well as in other sciences. But it is a certain variety of metaphysical holism that is more closely related to nonseparability. What is at issue here is the extent to which the properties of the whole are determined by the properties of its parts: property holism  (Section 4)  denies such determination, and thereby comes very close to a thesis of nonseparability. In turn, nonseparability can be analyzed either as state nonseparability  (Section 5),  or as spatiotemporal nonseparability  (Section 6).  By and large, a system in classical physics can be analyzed into parts, whose states and properties determine those of the whole they compose  (Section 7).  But all hands agree that the state of a system in quantum theory resists such analysis. The quantum state of a system specifies its chances of exhibiting various properties on measurement. In ordinary quantum mechanics, the most complete such specification is given by what is called a pure state. Even when a compound system has a pure state, some of its subsystems may not have their own pure states. Emphasizing this characteristic of quantum mechanics, Schrödinger described such component states as “entangled”  (Section 8).  Superficially, such entanglement of states already demonstrates nonseparability. At a deeper level, it has been maintained that the puzzling statistics that arise from measurements on entangled quantum systems either demonstrate, or are explicable in terms of, holism or nonseparability rather than any problematic action at a distance  (Sections 8,  9). The Aharonov-Bohm effect  (Section 10)  also appears to exhibit action at a distance, as the behavior of electrons is modified by a magnetic field they never experience. But this effect may be understood instead as a result of the local action of nonseparable electromagnetism. And indeed, this type of holism is not rooted in quantum mechanics or entanglement: though the measurement of the Aharonov-Bohm effect uses quantum ingredients, it builds on the existence of non-separable quantities in classical gauge theory (e.g. electromagnetism) in a vacuum. But even focussing on theories without gauge symmetry, we can obtain puzzling correlations between distant simultaneous measurements in the vacuum, according to quantum field theory  (Section 12).  A form of quantum theory used to study them represents systems by algebras of operators with new kinds of states defined on them, thereby making room for failures of state and system separability with no analogs in ordinary quantum mechanics. String theory  (Section 13)  is an ambitious research program in the framework of quantum field theory. According to string theory, all fundamental particles can be considered to be excitations of underlying non-pointlike entities in a multi-dimensional space. The particles’ intrinsic charge, mass and spin may then arise as nonseparable features of the world at the deepest level.