In computer-based interactive environments meant to support learning, students must bring a wide range of relevant knowledge, skills, and abilities to bear jointly as they solve meaningful problems in a learning domain. To function effectively as an assessment, a computer system must additionally be able to evoke and interpret observable evidence about targeted knowledge in a manner that is principled, defensible, and suited to the purpose at hand (e.g., licensure, achievement testing, coached practice). This article describes the foundations for the design of an interactive computer-based assessment of design, implementation, and troubleshooting in the domain of computer networking. The application is a prototype for assessing these skills as part of an instructional program, as interim practice tests and as chapter or end-of-course assessments. An Evidence Centered Design (ECD) framework was used to guide the work. An important part of this work is a cognitive task analysis designed (a) to tap the knowledge computer network specialists and students use when they design and troubleshoot networks and (b) to elicit behaviors that manifest this knowledge. After summarizing its results, we discuss implications of this analysis, as well as information gathered through other methods of domain analysis, for designing psychometric models, automated scoring algorithms, and task frameworks and for the capabilities required for the delivery of this example of a complex computer-based interactive assessment.