The content areas provide rich contexts for developing vocabulary. This article presents some principles and a lesson model--the learning cycle--that can be used to develop vocabulary while building understanding in science. Because science instruction and the learning cycle model promote learning in real-world contexts, they provide students with a motivating environment in which to learn new words. The engage, explore, develop, and apply phases of the learning cycle create opportunities for students to encounter and use words in multiple contexts, making it more likely that they will make the terms part of their own receptive and expressive vocabularies. The article provides examples of effective vocabulary instructional strategies that teachers can use in the learning cycle. Many strategies have features that allow teachers to add new terms and modify understanding as new learning occurs. A sample learning cycle lesson plan emphasizing vocabulary development within a science class is provided. (Contains 6 figures and 1 table.)