Although changes in stress systems are not as meticulously described and discussed as other aspects of phonology, they are recorded in the historical literature. Stress is related to quantity and weight and any change thereof may lead to an alteration of stress patterns. Usually, a change in the direction of stress assignment is rare, although abrupt changes from left-edge to right-edge stress or vice versa are known to happen. Causes for such changes are frequently assumed to be rooted in language contact. This chapter argues that stress patterns are surprisingly pertinacious, and that universal metrical preferences and constraints govern possible and impossible prosodic shifts. Rather than external influence alone, the chapter argues that acquisition and learnability can account for the data more coherently. It first considers general issues in the change of stress parameters and then focuses on the history of English, to exemplify some possible types of change on the basis of the prosodic changes that have occurred in that language.