Book excerpt: Xcos is a very powerful and open source block-based modeling and simulation system for dynamical systems. Its capabilities are comparable to commercially available block-based modeling and simulation tools, including Simulink(R), one of the most popular commercial tool. Xcos is useful for modeling continuous and discrete dynamical systems. Further, it provides facilities to seamlessly integrate continuous and discrete components in a single model, making it capable to handle hybrid dynamical systems. Xcos provides a modular approach to model complex dynamical systems using a block diagram editor. Xcos contains a rich library of commonly used blocks, arranged in various palettes for the convenience of searching them, for elementary operations needed to construct models of many dynamical systems. These blocks can be dragged and dropped into the model editor to create a simulation model. For advanced users, Xcos provides facilities to create new blocks and to create their own libraries to further extend the capabilities of Xcos. Since Xcos is available free of cost to everyone across the globe and is continuously upgraded by a strong team of open source developers, it is suitable for all undergraduate students, researchers, professors and professionals in any field of Science and Engineering. Further, many commercial developers are also using it to reduce their project cost and has reported many successful applications.Starting from the basic concepts, the book gradually builds advanced concepts, making it suitable for freshmen and professionals. The Xcos models of all the examples included in this book are available at https: //github.com/arvindrachna/Introduction_to_Xcos. The book consists of the following 15 chapters: Chapter 1: Introduction to XcosChapter 2: Sources PaletteChapter 3: Sinks PaletteChapter 4: Mathematical Operations PaletteChapter 5: Matrix Operation PaletteChapter 6: Signal Routing PaletteChapter 7: Event Handling PaletteChapter 8: Integer PaletteChapter 9: Continuous Time Systems PaletteChapter 10: Discrete Time Systems PaletteChapter 11: Discontinuities PaletteChapter 12: Port and Subsystem PaletteChapter 13: User-Defined Functions Palette and Construction of a New BlockChapter 14: Illustrative Solutions of Differential Equations using XcosChapter 15: Modelica based blocks in Xcos