This activity book is aimed to provide an introduction to the simulation practice in Engineering using Modelica. To this end, we propose a series of thirteen independent hands-on assignments of increasing complexity. Each assignment contains the description of a system and a mathematical model of the system's behavior. The proposed task often consists in describing this mathematical model in the Modelica language and simulate it. In some assignments, the system's behavior is described as an atomic model, without internal structure. Some other assignments ask to design and implement a model library, and to compose the system model by instantiating and connecting components from this model library. Before start working with this activity book, it is advisable to read its companion theory book: a free e-book entitled "Modeling and simulation in Engineering using Modelica", written by Alfonso Urquía and Carla Martín, and published by Editorial UNED in 2018