Existing research has focused on supply-side explanations for the timing of major technological transitions in an industry. Work in this tradition has argued that shifts to a radically new technology occur when the evolutionary path of the old technology is approaching diminishing returns. This paper suggests that the timing of technological discontinuities is driven as much by the evolution of demand-side preferences - in particular, preference discontinuities - as by the evolution of the technology. I introduce the concept of preference trajectories, cycles of incremental and discontinuous changes in preferences, ad an analog to technological trajectories and explore the interrelationships between the two. This perspective is illustrated by data on the evolution of both technology and preferences over a period of 100 years in the typesetter industry.