This chapter presents an overview of the conceptual toolkits used to theorise relationships between ECEC settings and families in academic journal articles, published in English, between the years 2000–2010 and 2021–2022. The reconstructed overview of the theories employed by researchers from different regions of the world creates a base for classification of the theories as positivistic (i.e., interested in measuring PI for prediction and control of academic achievement), interpretative (i.e., aiming at deeper contextual understandings of the perspectives of all social actors that have a part PI), or critical (i.e., delving into the socio-economic conditions and power relations constituting diverse understandings of the world of PI, in conjunction with the desire for change). The chapter concludes with an outline of the theories discussed in further chapters, which are of an interpretive and critical nature and embrace the understanding of more-than-parental involvement presented in Chap. 1.