Three MAPK (Mitogen Activated Protein Kinase) intracellular signalling cascades, the ERK (Extra-cellular-Regulated-Kinase) pathway, the p38 pathway and the JNK (c-Jun N-terminal Kinase) pathway function in mammalian cells. In some forms of human breast cancer and in many experimental models of breast cancer progression, signalling through the ERK pathway, in particular, has been implicated as being important. We review the influence of ERK activity on organised association of mammary epithelial cells in 3-D, and in models of breast cancer cell invasion. We assess the importance of EGFR family signalling through ERK in models of breast cancer progression and the influence of ERK on its substrate, the estrogen receptor (ER), in this context. In parallel, we consider the importance of these ERK-centred signalling cascades during the cycle of mammary gland development. Although less well studied, we highlight the instances of signalling through the p38 and JNK pathways with breast cancer progression and mammary gland development.