This book is for scientists and engineers involved in the definition and development of space science missions. The processes that such missions follow, from the proposal to a space agency, to a successful mission completion, are numerous. The rationale behind approval of a mission, its definition and the payload that it will include are topics that cannot be presented in undergraduate courses. This book contains contributions from experts who are involved in today's space missions at various levels. Chapters cover mission phases and implementation, launchers and cruise strategies, including gravity assist maneuvers and different thrust scenarios. The payload needed for remote sensing of the Universe at various wavelengths and for in-situ measurements is described in detail, and particular attention is paid to the most recent planetary landers. Whilst the book concentrates on the ESA program Cosmic Visions, its content is relevant to space science missions at all space agencies.