The first member of the Wnt-family ligands was identified 30 years ago as a factor in mouse mammary tumours whose expression was deregulated due to the promoter activity emanating from the proximal integration of the Mouse Mammary Tumour Virus genome (Nusse and Varmus, Embo J 31:2670–84, 2012). The Wnt-ligands invoke a number of molecular-genetic signalling cascades fundamental to the patterning of developing tissues and organs during embryogenesis as well as during postnatal development. The Wnt-signalling cascade that controls the activities of β-catenin and the T-cell Factor (Tcf)/Lympoid enhancer factor (Lef1) plays a fundamental role in control of all stages of embryonic mammary gland development. We provide here a brief overview of the known aspects of Wnt-signalling activities in the embryonic mammary gland and its interactions with other signalling cascades in this developing tissue.