This qualitative study explored the nature of informal learning in professional instructional designers' everyday work activities. Based on intensive interviews with six full-time practitioners, and using a hermeneutic form of data analysis, this study produced seven themes concerning the practices, tensions, and ironies associated with this largely unexplored aspect of instructional design. Specific themes concerned the resources, significance, and outcomes of informal learning, in addition to professional tensions and ironies (e.g., the observation that informal learning simultaneously fosters professional survival and professional vulnerability). Implications are discussed, such as how informal learning among professionals can be facilitated via organizational support and more practically-oriented scholarly work, as well as designers' own efforts to pursue informal learning in the midst of practice.