This article describes a blueprint for an online learning environment that is based on prominent instructional design and assessment theories for supporting learning in complex domains. The core of this environment consists of formative assessment tasks (i.e., assessment "for" learning) that center on professional situations. For each professional situation, three levels of situational complexity are defined, and within each of these three levels, tasks are offered that differ in the degree of support offered to the learner. This environment can support (beginning) professionals in complex domains in gaining insight into the available repertoire of behavior in professional situations, as well as into the quality and effectiveness of that behavior (assessment criteria), while simultaneously helping them to develop insight into the standards that their own behavior should (eventually) match.