The Single Market is one of the cornerstones of the European Union. The idea to transform it into a Digital Single Market (DSM) was outlined several years ago. The EU has started different initiatives to support this transformation process. One of them is the program Horizon 2020 to support the process from a technical point of view. In parallel to this, initiatives were started to set up a sound legal framework for the DSM. The Single Digital Gateway Regulation (SDGR) is an outcome of these initiatives. The key aspect of the SDGR is the underlying Once-Only Principle (OOP), outlining that businesses and citizens in contact with public administrations have to provide data only once. “The Once-Only Principle Project (TOOP)” is the EU-funded project initiated for research, testing, and implementation of the OOP in Europe. The authors give an overview of the research questions of the different parts of TOOP. Besides that, they introduce the other chapters of this book and what the reader can expect as the content of them.