The need for e-learning systems that support a diverse set of pedagogical requirements has been identified as an important issue in web-based education. Until now, significant research and development effort has been devoted to aiming towards web-based educational systems tailored to specific pedagogical approaches. The most advanced of them are based on the IEEE Learning Technology Systems Architecture and use standardized content structuring based on the ADL Sharable Content Object Reference Model (SCORM) in order to enable sharing and reusability of the learning content. However, sharing of learning activities among different web-based educational systems still remains an open issue. On the other hand, existing Learning Management Systems (LMS) provide authoring tools that are only tailored to the capabilities of the specific system in hand. The open question is how web-based educational systems should be designed in order to enable reusing and repurposing of learning activities. In this article we first discuss the limitations of the current state-of-the art learning systems and authoring tools and investigate the use of the Learning Design framework as a mean to address those limitations. Then, we present a high-level architecture of a SCORM-compatible authoring and runtime system that utilizes the Learning Design principles to provide the means for designing activity-based learning systems. Finally, we discuss the use of ASK-LDT, an authoring system developed based on the proposed architecture, in the design of complex learning activities. (Contains 10 figures.)