Tropos is a software development methodology founded on the key concepts of agent-oriented software development. Specifically, Tropos emphasizes concepts for modelling and analysis during the early requirements phase. This phase precedes the prescriptive requirements specification of the system-to-be. In this paper, we present the Tropos metamodel starting from the basic concepts of actor, goal, plan, resource and social dependency and then we illustrate its use by introducing an extension intended to introduce concepts for modelling security concerns. We also sketch the Tropos modelling environment and compare with the metamodels of other software development methodologies.