To specify its intention clearly, a pointcut definition must specify the join point where to adapt a crosscutting functionality and the applicability condition of the crosscutting functionality (if any), and to realise modular abstraction of a crosscutting functionality an advice must abstract only the crosscutting functionality. A state-dependent crosscutting concern is an aspect whose applicability of its advice at a matched join point is statedependent, and call graph-based AOP approaches are insufficient for a state-dependent crosscutting concern because they do not allow specification of the state-dependent applicability condition in the pointcut definition. Event-based AOP approaches expose sequences of event executions as join points. Therefore, the state-dependent applicability condition of an aspect's advice can be specified in a pointcut definition by stating the sequence of event executions that brings the base program into the state when the advice is applicable. However, this means the programmer must know the sequence of event executions that brings the base program into the state. This thesis proposes a state-based AOP approach that exposes state and state transition join points that allow a state-dependent applicability condition to be clearly specified in the pointcut definition simply by specifying the state or the state transition when the advice is applicable, as implemented in the proof-of-concepts prototype language, TinyState.