This article presents the Evaluand-oriented Responsive Evaluation Model (EREM), a comprehensive evaluation model to be used in the evaluation of a CSCL system. The model relies on a responsive evaluation approach to provide potential evaluators with a practical tool to evaluate multiple criteria and episodes. Introduction: Initial Stitches In 1927 theoretical physicist Werner Heisenberg articulated his uncertainty principle. Roughly speaking, it states that the position and velocity of an object cannot both be measured exactly at the same time, and that the concepts of exact position and exact velocity together have no meaning in the small scales of atoms and subatomic particles. We find the principle useful to illustrate the uncertainty involved in the evaluation of CSCL programs, courses, learning strategies, projects, and technological tools. Evaluation is intrinsic to human life, hence complex and intricate. We are always balancing things, decisions, opinions as to whether or not ask something, do something, etc. Stufflebeam (1971) and Cronbach (1980) defined the main goal of formal evaluation as improvement in decision-making. But there are many purposes even for a single evaluation study (Stake, 2003). CSCL is an interdisciplinary field different from other applications of ICT to learning and/or collaboration. This difference is its emphasis on group learning. Its theoretical foundations consider knowledge as a learner construction promoted by the interaction of learners with their social and physical environment. For Koschmann (2002) CSCL is "a field of study centrally concerned with meaning and the practices of meaningmaking in the context of joint activity and the ways in which these practices are mediated through designed artifacts." The design and enactment of CSCL systems and scenarios is inherently complex, with a wide mix of disciplinary perspectives. Teachers, curriculum designers, evaluators, students, and technology developers work together. Comprehensive evaluation of these systems is challenging. Treleaven (2003) argues that "evaluation in these contexts challenges traditional approaches to evaluation and requires new theoretical frameworks to guide analysis and interpretation." The different conceptions of the CSCL field, the strong social component that defines it, and the multitude of value questions surround the evaluator in uncertainty. Many criteria standards issues and interpretations of a complex system remain open even at the end of the study. In this paper we present an evaluation model to be used by CSCL practitioners and evaluation specialists called Evaluand-oriented Responsive Evaluation Model (CSCL-EREM). The model is framed within what Lincoln and Guba (1989) have called the "Fourth generation of evaluation". Evaluators respond to participants´activity more than measuring them. The model orients the evaluator to the systemic activity, noting the uniqueness and the commonality of the evaluand to be evaluated. The evaluator is responsive to key issues and problems recognized by pa...