Space exploration and commercial activity off-world has its skeptics as well as its enthusiasts. What does seem to be clear, however, is that such activity has increased and is set to expand further, and dramatically so, during the present century. This book explores some of the ethical issues which have already started to arise and it explores the prospects for our medium-range future: Can terraforming of other worlds succeed? Would it be defensible? Should there be limits to mining in space? Do lifeless planets have an 'integrity' which we ought to respect? Could indigenous micro-bacteria have any special intrinsic value? Do we have a duty to extend human life? The text then moves onto a treatment of the ethics of sending world-ships on inter-stellar journeys and the unpredictable risks associated with seeding other worlds with rudimentary forms of life. Throughout, the book is as much about our humanity as it is about space. (And here, a shared humanity is not reducible to species membership.) It concludes with an attempt to explore the connection between our belonging to a single home planet and our sense of belonging to a single moral community.