Outside school, people typically learn during their experiences while addressing desired goals. The Goal-Based Scenario (GBS) framework describes computer-based learning environments that exploit this simple fact. In this article, we propose a structure and a set of design criteria for learn-by-doing environments that enable students to work towards desired goals. A key issue we address is the content to be taught by GBSs. Because skills are the form of knowledge that, when applied, enable students to achieve valued goals, we argue that GBSs should be designed to teach a set of target skills required to achieve a specified goal. Two programs we built prior to specifying GBSs but motivated by many of the same ideas will be analyzed according to the proposed principles. We conclude by briefly describing tools currently under development to facilitate the construction of GBSs.