This study employed a qualitative research design to investigate informal learning among practicing instructional designers. Prior research has examined how instructional designers spend their time, make decisions, use theory, solve problems, and so on, but no published research has explored the nature and role of informal learning in instructional design work. Based on intensive interviews of practitioners in the field, this study produced eight themes organized according to two metathemes: (a) the nature of informal learning in instructional design practice and (b) instructional design as informal learning. Specific themes concerned what instructional designers learn through informal practical experience, how they learn it, and the meaning of this kind of learning for various aspects of their work. Overall, these results suggest that informal learning is a vital part of instructional design practice and that design itself can be thought of as a specialized type of informal learning. Other conclusions regarding informal learning in design are discussed and future directions for research are offered.