Software-intensive systems are systems of systems that rely on complex interdependencies among themselves as well as with their operational environment to satisfy the required behavior. As we integrate such systems to create information infrastructures that are critical to the quality of our lives and the businesses they support, the need to effectively predict, control and evolve their behavior is ever increasing. To deal with their complexity, an important first step is to understand and model software-intensive systems, their environments and the interdependencies among them at different levels of abstractions from multiple dimensions. In this paper, we present an Ontology-based Active Requirements Engineering (Onto-ActRE [onto-..kt..r]) framework that adopts a mixed-initiative approach to elicit, represent and analyze the diversity of factors associated with software-intensive systems. The Onto- ActRE framework integrates various RE modeling techniques with complementary semantics in a unifying ontological engineering process. We also present examples from the practice of our framework with appropriate tool support that combines theoretical and practical aspects.