The ?eld of Bose–Einstein condensation in atomic gases has been full of s- prises.Whathappenedafterthe?rstrealization ofBose–Einstein condensates in 1995 has far exceeded the vision of their creators. Originally, I expected interesting studies of equilibrium properties and dynamics, but the Bose– Einstein condensate became a pristine platform for a host of scienti?c studies. The condensate turned out to be a well-controlled medium for nonlinear wave phenomenaincludingfour-wavemixing,darkandbrightsolitons,anddynamic instabilities. Rotating condensates gave access to the rich physics of vortices and vortex lattices. The availability of di?erent hyper?ne states led to mul- component or spinor condensates, and optical lattices opened up yet another major frontier which is still rapidly expanding. Feshbach resonances and c- densation of fermion pairs will provide interesting phenomena to study for many more years to come. This rapid development of the ?eld has been fueled by a unique int- play between experiment and theory. Although experimentalists had the lead in realizing new systems, many phenomena were ?rst predicted and then observed. The present book re?ects this symbiosis by a balance of theoretical and experimental contributions. Twelve years after its beginning, the ?eld has reached a degree of maturity which is much better represented by a comp- hensive book than by a collection of original articles. I therefore expect this book not only to provide a valuable introduction to many young researchers who are joining this ?eld, but also serve as a reference for further studies of nonlinear phenomena in this rich system.