The discussion of the reduction of mental states to brain states is placed in the broader context of reduction in other scientific disciplines such as chemistry, physics and biology. This is important in achieving a perspective on what sort of business reduction is and in seeing that reductions are primarily transitions between theories and only derivatively relations between phenomena. It also reveals that though reducing theories sometimes absorb the old theory as largely correct, more often the old theory is substantially modified and revised and sometimes it is replaced outright. How much of the old theory survives in the reducing theory depends on its empirical integrity and whether its basic categories are empirically sound. The reduction of psychology to neuroscience is considered in this light and it is suggested that psychology may be substantially revised or even replaced by a reduction to neuroscience.