This is a collection of twenty outstanding lectures ranging across the most active areas of astronomical research. In the series distinguished scientists, mostly British and American, and including three Nobel Laureates, present their work and perspectives on current developments.The lecturesthemselves vary in length and content, and while they invite scrutiny by the specialist, the bulk of the material is well within the reach of a third year undergraduate. The originality of thought, which is presented in vibrant and sometimes controversial terms, makes the Lectures a unique recordof trends in astrophysics during the last two decades.