The geometric and algebraic aspects of two-dimensional homotopy theory are both important areas of current research. Basic work on two-dimensional homotopy theory dates back to Reidemeister and Whitehead. The contributors to this book consider the current state of research beginning with introductory chapters on low-dimensional topology and covering crossmodules, Peiffer-Reid identities, and concretely discussing P2 theory. The chapters have been skillfully woven together to form a coherent picture, and the geometric nature of the subject is illustrated by over 100 diagrams. The final chapters round off neatly with a look at the present status of the conjectures of Zeeman, Whitehead and Andrews-Curtis.