While intergenerational upward mobility is often regarded both a "natural" and a desirable element of democratic and open societies, downward mobility has often been considered "unnatural" and a result of individual failure. Nevertheless, considerably less research has been done on explanations to downward mobility than to upward. A central research question in this project is to what extent factors affecting upward mobility also can contribute to explaining downward mobility. Even though downward mobility of course cannot be perceived as upward mobility inverted, the thesis is that new knowledge can be generated through a closer integration of the phenomena than has usually been the case. It can be argued that research into explanations of upward mobility is of special importance with reference to social justice. The point of departure for this project is however that thorough understanding of intergenerational mobility requires the study of processes in both directions. In addition to the fact that knowledge in causes of downward mobility is of interest in itself, it can also uncover yet unknown explanations to the still significant differences in life chances related to class of origin. Access to unique longitudinal interview data in combination with information from registers implies very good possibilities of empirical analyses.