Aspect-Oriented Programming (AOP) is a new programming paradigm that promotes advanced separation of concerns by introducing a new modular unit, called aspect, for the modularization of crosscutting concerns. As an emerging programming paradigm, there are many different approaches to AOP, with distinct and varying sets of abstractions and composition mechanisms, specific terminology, properties and language constructs. In this thesis, we shift the focus from the mechanisms that support the aspect-oriented programming technology to the concepts and properties that characterize it as an emerging paradigm for software development. In particular, we focus on how these concepts and properties can be exploited at an early design stage to build systems that are easier to comprehend, evolve and reuse. This thesis addresses current issues related to aspect-oriented design and modeling, and proposes: (i) the aspect model, an unifying conceptual framework for AOP that provides consistent terminology and basic semantics for thinking about a problem in terms of AOP core concepts and properties, (ii) the aSideML, a modeling language for specifying and communicating aspect-oriented designs. The aSideML provides notation, semantics and guidelines that enable the designer to build models in which aspects are explicitly treated as first-class citizens, and (iii) the aSide metamodel, a logical model that defines the semantics of structural and behavioral models supported by the aSideML.