This paper explores the sequential structure of work processes in a task unit whose work involves high numbers of exceptions, low analyzability of search, frequent interruptions, and extensive delibration and that cannot be characterized as routine under any traditional definition. Yet a detailed analysis of the sequential pattern of action in a sample of 168 service interactions reveals that most interactions follow a repetitive, functionally similar pattern. This apparent contradiction presents a challenge to our theoretical understanding of routines: How can apparently nonroutine work display such a high degree of regularity? To answer this question, we propose a new definition of organizational routines as a set of functionally similar patterns and illustrate a new methodology for studying the sequential structure of work processes using rule-based grammatical models. This approach to organizational routines juxtaposes the structural features of the organization against the reflective agency of organizational members. Members enact specific performances from among a constrained, but potentially large set of possibilities that can be described by a grammer, giving rise to the regular patterns of action we label routines.