An early integration of performance considerations inside the software development process has been recognized during last years as an effective approach to speed up the production of high quality and reliable software. In this paper we propose a possible solution to address software performance engineering that evolves through the following phases: system specification using an augmented UML notation, creation of an intermediate performance context model, generation of an equivalent stochastic Petri netmodel whose analytical solution provides the required performance measures. We describe all the steps of the proposed approach and present ArgoPerformance, a tool we have developed that provides a complete graphical environment for software performance evaluation. We present a simple case study and validate our results against those obtained by other authors through simulation.