Non-diffracting wave packages or solitons have been the subject of intense study over the last three decades. In particular optical spatial solitons, for which diffraction is exactly balanced by self-focusing in a nonlinear medium, have strongly stimulated the research in the field of solitons in the 90’s, especially in photorefractive crystals. Such optical spatial solitons exhibit particle-like behavior in their interactions and stability properties, conserving energy and momentum, and the fascinating results obtained in this field have major consequences in many non-optical systems that can support solitons. This article explains the basic mechanisms that lead to soliton formation, in particular in photorefractive crystals, and gives a short overview of new directions like composite solitons, incoherent solitons formed with spatially incoherent light, and incoherent modulation instability. © Springer-Verlag Berlin Heidelberg 2001