Constructivism is a learning theory that states that people learn by actively constructing their own knowledge, based on prior knowledge. Many different perspectives exist on constructivist pedagogical principles and on how to apply them to instructional design. It is thus not only difficult to evaluate the conformity of existing learning systems with constructivist principles, it is also quite hard to ensure that a new learning system being designed will ultimately facilitate and stimulate constructivist learning. A critical characteristic often mentioned in learning systems is adaptability. That is, the ability to provide a learning experience that is continuously tailored to the needs of the individual learner. The present research aims to help designing truly constructivist and adaptive learning systems. For that purpose, it is necessary to clarify what constructivism entails in an operational manner: I propose a set of criteria for certain aspects of constructivism and use it both as guidelines for designing learning systems and for evaluating the conformity of learning systems with these constructivist principles. One facet often mentioned as being strongly relevant to constructivism is cognitive flexibility, meaning the ability to spontaneously restructure one's knowledge, in many ways, in adaptive response to radically changing situational demands. The claim I make in the present thesis is that the operational approach I proposed makes the design and use of adaptive learning environments supporting cognitive flexibility straightforward and effective. More specifically, the dissertation makes four main contributions to the interdisciplinary field of learning and e-Learning technology. Firstly, the thesis proposes operational criteria for cognitive flexibility and presents both justifications and examples of their use. The set of criteria may be used in different instructional situations for designing and evaluating conditions of learning. Secondly, on the basis of the criteria for cognitive flexibility, the thesis proposes an operational instructional design process and shows an example of its use. The process may also be applied in a variety of instructional situations for the design and use of learning systems fostering cognitive flexibility. Thirdly, the thesis introduces a new, open-source, domain-independent, Web-based adaptive e-Learning platform, named COFALE, and illustrates an example of its use. The platform may be used for designing adaptive learning systems supporting cognitive flexibility in various domains. And fourthly, the thesis reports on a preliminary evaluation of the example handled by COFALE with actual learners. The study provides a certain number of encouraging results for fostering cognitive flexibility by means of ICT-based learning conditions.