Agent-oriented models are frequently used in disciplines such as agent-oriented requirements engineering, requirements engineering and organizational process modelling. i* is currently one of the most widespread notations used for this purpose. Due to both its dissemination and its highly strategic nature, instead of a single definition, several groups have formulated different variants, which define in a slightly different way the basic concepts of the language, as well as propose particular constructs that fit into the particular interests of these groups. In this chapter we first review these proposals and then we present a reference model that establishes the concepts and relationships that are fundamental in i*. The reference model is expressed as a UML class diagram (with OCL constraints) together with a vocabulary of the presented terms. Most of the included concepts are common to the seminal i* proposal, the GRL and the diverse variations used in the context of the Tropos methodology. For those concepts that are not shared by these analysed variants, the reference model tries to reconcile the different views whenever possible. The reference model allows determining the differences of any existing i* variant with respect to the model, and also to know how much different a new variant would be from the core of i*. The variants can be expressed by applying some refactoring operations on the UML class diagram. We illustrate this situation by applying refactoring to some of the analysed proposals.