In their paper, Martin Bujard and Detlev Lück point out that the decline of birth rates in low fertility countries such as Germany is driven by two demographic phenomena: (1) the decrease of large families and (2) the increase of childlessness. Both phenomena play different roles at different historical stages, and there are different causes for each of the two. Therefore, the aim of the K&K project is to describe and to explain in a differentiated manner the phenomena of childlessness and large families. Current theories for their causes contain gaps due to a lack of explanation as to which demographic phenomenon the theories are based on. In this paper, the authors develop the theoretical foundations for understanding childlessness as well as the transition to having a third child. Furthermore, the authors distil arguments and conceptualise an analytical framework that combines macro, meso and micro levels with the perspectives of couples and life course. Bujard and Lück come to the conclusion that the decline of large families is mainly caused by cultural factors. The pervasiveness of the normative model of the two-child family, as well as the low social recognition of large families, lead to the overall decrease in families with three or more children. Childlessness, on the other hand, is most frequent when common postmodern factors meet insufficient changes in society and on the labour market. Frequently, this causes the first child to be born later in a mother's life, as well as an overall increase in opportunity costs for raising a child in the first place.