This article investigates the costs and benefits of the increased use of modular or unitized qualification designs through a case study of the GCE A-level science curriculum in England. Following a brief review of the development of modular A-levels, the various proposed advantages of modularity--short-term goals and regular feedback, flexibility in curriculum design, and improved progression possibilities--are counterpoised by arguments about the disadvantages--such as fragmentation of knowledge and more instrumental approaches to assessment and learning. The article argues that on balance the costs of the move to modularization in terms of the impact on teachers' capacities to help young people understand science outweigh the perceived benefits of improved examination success rates. Given this balance we account for the growing popularity of modular approaches using a path dependency model and increasing returns process which combine features of the English educational landscape, in particular narrow accountability systems, to the increasing desirability of modular approaches to curriculum design for learners, teachers and educational organizations. (Contains 4 notes.)