Xmipp is a specialized software package for image processing in electron microscopy, and that is mainly focused on 3D reconstruction of macromolecules through single-particles analysis. In this article we present Xmipp 3.0, a major release which introduces several improvements and new developments over the previous version. A central improvement is the concept of a project that stores the entire processing workflow from data import to final results. It is now possible to monitor, reproduce and restart all computing tasks as well as graphically explore the complete set of interrelated tasks associated to a given project. Other graphical tools have also been improved such as data visualization, particle picking and parameter "wizards" that allow the visual selection of some key parameters. Many standard image formats are transparently supported for input/output from all programs. Additionally, results have been standardized, facilitating the interoperation between different Xmipp programs. Finally, as a result of a large code refactoring, the underlying C++ libraries are better suited for future developments and all code has been optimized. Xmipp is an open-source package that is freely available for download from: http://xmipp.cnb.csic.es. (C) 2013 Elsevier Inc. All rights reserved.