This work is primarily the proceedings of a workshop devoted to the internal structure of blackholes and spacetime singularities. The chapters are mainly written versions of the talks given at the workshop. However, the authors have have written extended introductions to their contributions, so that a reader, well versed in gravitational theory, but unaware of the current research in this subfield, should be able to follow the text. The book should be suitable for graduate students who wish to familiarize themselves with the various aspects of the structure of black holes, and for resarchers who wish to learn about in subfields other than their own. Topics covered include: the nature of singularities (both null and spacelike) and the spacetime inside black holes coupled to various matter fields in classical and semiclassical gravity, as well as in alternative gravity theories; the cosmic censorship hypothesis and its connection to black hole interiors; cosmological singularities; and their relation with black holes singularities; and black hole entropy and evporation.