Constrained skills theory is a reconceptualization of reading development that suggests a continuum of skills, with some, such as letter knowledge and decoding abilities, more tightly constrained than others, such as phonological awareness and oral reading fluency. The most constrained skills consist of a limited number of items and thus can be mastered universally within a relatively short time frame. The least constrained skills, vocabulary and comprehension, are learned across a lifetime, broad in scope, variable among people, and may influence many other skills. This article addresses the important implications of this theory for classroom practice, curricula, and assessment. Finally, cautions are issued regarding the potential to overemphasize the assessment and instruction of constrained abilities in an effort to enhance the likelihood of the successful long-term acquisition of unconstrained abilities.