In order to confine the huge field, we shall focus the discussions on finite systems wherefrom we take the examples of applications. This covers nuclei, atoms, molecules and clusters. The idea of this book is to concentrate first on the generic, robust and simple, approaches and show how they apply to several domains across the specific disciplines. On the other hand, we aim to establish contact with actual research by carrying forth some examples up to realistic applications, attacked with help of a set of simple and still general codes provided at an online repository linked to the book. This latter aspect emphasizes our intention to guide the reader in "practizing" the tools presented in the book, both at schematic and realistic levels.