Ensuring the effectiveness factor of usability consists in ensuring that the application allows users to reach their goals and perform their tasks [4]. The interactive application shall implement all of the needed functionalities and these functionalities shall be accessible when needed (e.g. made available to the user in an order that is compatible with the one needed to reach the goals). Ensuring the effectiveness of an interactive application thus requires an explicit description of users' goals and tasks. Task models is one of the very few means for explicitly and exhaustively describing user tasks at design time. The only mean for ensuring that this need is met requires an explicit description of users' goals and tasks. How to understand these goals and tasks and how to demonstrate that an interactive application supports them all is the main objective of this tutorial.