This invaluable book provides both an introduction to the rudiments of perturbative string theory and a more detailed introduction to the more current topic of D-brane dynamics. The approach taken is pedagogical, with much of the technical detail streamlined. It is the concise nature of this rapid, but highly coherent, introduction to the subject that distinguishes it from other books on string theory or D-brane dynamics. The third edition has been revised to include a new chapter on the recent topic of string theory in electromagnetic fields. The material is based on courses in theoretical high-energy physics delivered by the author at various summer schools, so its level of appropriateness has been tested and has been resoundingly well received.