This book is based on lectures given at a summer school held in Nordfjordeid on the Norwegian west coast in August 2002. In the little town with the sp- tacular surroundings where Sophus Lie was born in 1842, the municipality, in collaboration with the mathematics departments at the universities, has established the “Sophus Lie conference center”. The purpose is to help or- nizing conferences and summer schools at a local boarding school during its summer vacation, and the algebraists and algebraic geometers in Norway had already organized such summer schools for a number of years. In 2002 a joint project with the algebraic topologists was proposed, and a natural choice of topic was Motivic homotopy theory, which depends heavily on both algebraic topology and algebraic geometry and has had deep impact in both ?elds. The organizing committee consisted of Bjørn Jahren and Kristian Ran- tad, Oslo, Alexei Rudakov, Trondheim and Stein Arild Strømme, Bergen, and the summer school was partly funded by NorFA — Nordisk Forskerutd- ningsakademi. It was primarily intended for Norwegian graduate students, but it attracted students from a number of other countries as well. These summer schools traditionally go on for one week, with three series of lectures given by internationally known experts.