This chapter presents the results of 25 years of research on modeling and using context in realworld applications. The first milestone was an operational definition of context relating it to a focus (problem-solving, decision-making, task realization). The focus provides context with links between context and knowledge, a division between contextual knowledge and external knowledge with respect to a focus, and a dynamic linked to reasoning evolution. Conversely, context makes focus more explicit. The second milestone was a context-based representation of decision trees as contextual graphs with contextual elements (expressing the focus) instead of chance or event nodes. The operational definition of context opens up a conceptual framework that can be associated with an implementation framework. The third milestone was the development of a context-based representation formalism that led to the software Contextual Graphs (CxG) that has been used in about 20 real-world applications. The fourth milestone concerns an extension of the CxG representation formalism for representing group activity with a cyclic use of a CxG for simulation purposes. The formalism now allows a modeling of "nonlinear" behaviors of a group (e.g. interaction, negotiation, etc.) as well as of a user (e.g. checking alternatives). The fifth milestone opens the way to Context-based Intelligent Assistant Systems.