The lecture, titled 'Black Holes and Young Universes,' was first given at the University of California, Berkeley. Supplemented by Stephen Hawking's reflections on this and related topics, it is part of this collection, comprised of 13 essays and an extensive interview. On these pages, the most renowned astrophysicist of our time discusses the most probable models for the structure of the Universe. This continues the dialogue with the reader begun in A Brief History of Time. The author discusses imaginary time, how black holes can give rise to young universes, and the efforts of scientists, initiated by Albert Einstein, to find a unified field theory that could make the macrocosm predictable.