This book is intended for everyone who does scientific work with images and is interested in knowing what can be achieved with ImageJ. The introductory part is a very broad introduction to the field of digital image processing, while the latter chapters are devoted to more complex analysis techniques in addition to an introduction to the macro and plugin system, that we hope will clarify these concepts to everyone interested in automating their analysis procedures or implementing their own image analysis algorithms. We have tried to accomplish a good balance between a step-by-step guide for non-advanced users and a nice introduction for developers who might want to use ImageJ as the development platform of their choice.