 In traditional approaches to quantum measurement theory a central role is played by the projection postulate, which asserts that upon measurement the state of a physical system undergoes a discontinuous change: it is projected (“collapses”) onto an eigenstate corresponding to the measurement result. This postulate leads to many difficulties: When exactly does a collapse takes place? What defines a “measurement” as opposed to an ordinary physical interaction? The postulate is especially worrying when applied to entangled compound systems whose components are well-separated in space. For example, in the Einstein-Podolsky-Rosen (EPR) experiment there are strict correlations between two systems that have interacted in the past, in spite of the fact that the correlated quantities are not sharply defined in the individual systems. The projection postulate in this case implies that the collapse resulting from a measurement on one of the systems instantaneously defines a sharp property in the distant other system. (See the discussion of the collapse or projection postulate in the entry on  philosophical issues in quantum theory.)