 If quantum theory is meant to be (in principle) a universal theory, it should be applicable, in principle, to all physical systems, including systems as large and complicated as our experimental apparatus. It is easy to show that linear evolution of quantum states, when applied to macroscopic objects, will routinely lead to superpositions of macroscopically distinct states. Among the circumstances in which this will happen are experimental set-ups, and much of the early discussions focussed on how to construe the process of measurement in quantum-mechanical terms. For this reason, the interpretational issues have come to be referred to as the measurement problem. In the first decades of discussion of the foundations of quantum mechanics, it was commonly referred to as the problem of observation.