This introductory chapter provides an overview of Easy Java/JavaScript Simulations (EjsS), the high-level modeling and authoring tool that we use to teach modeling and simulation. We first describe the installation process and the file structure of our program, and then run the EjsS console to get our first program on the screen. We describe how EjsS supports different programming languages for the modelling process. Subsequent chapters in this document provide a step-by-step introduction to the different parts of the EjsS graphical user interface, as well as to the modelling process. We finally instruct you how to download existing models created with EjsS from our on-line digital libraries.