Luminal epithelial cells in the mammary gland require two types of signals to proliferate: soluble signals (growth factor signals) and signals from the extracellular matrix (ECM). The composition of the ECM is sensed by adhesion receptors such as integrins. Integrins modulate cell behaviour and play a key role in cell cycle entry. Altered integrin expression and signalling has been associated with breast cancer and studies using mouse mammary epithelial cells (MECs) have shown that the absence of β1-integrin induces growth arrest. However, it is not completely understood how integrins transduce the signals from the plasma membrane to the nucleus to induce cell cycle entry. Thus, the first aim of this project was to determine how β1-integrin controls proliferation in MECs. I established a model to study the effects of depleting β1-integrin using the FSK7 mammary epithelial cell line. The proliferation defect observed in this β1-integrin knockdown model was rescued by expressing a constitutively active Rac1 or Pak. Moreover, inhibiting Rac1 or Pak prevented normal proliferation in MECs in a similar fashion as β1-integrin depletion. Furthermore, in this thesis I have identified the complex comprised of Src, paxillin and p130Cas as a potential link between β1-integrin and Rac1. These results provide an insight into the mechanism that regulates proliferation downstream of β1-integrin. During breast cancer initiation, β1-integrin signals are disrupted. This indicates that additional signals must be driving proliferation during tumorigenesis. Therefore, the second aim of this project was to test whether expression of breast oncogenes can overcome the proliferation defect present in β1-integrin null cells. In order to do so, an oncogenic ErbB2, a constitutively active form of Akt (myrAkt) and the Notch1 intracellular domain (NICD) were transfected in the β1-integrin knockdown MECs. The results showed that ErbB2 overcomes the need for β1-integrin by signalling to Pak. NICD does not require β1-integrin to drive proliferation by an unknown mechanism. Expression of myrAkt did not restore normal levels of proliferation in β1-integrin depleted MECs. This finding suggests that Akt is not sufficient to induce cell cycle entry by itself and instead, both Akt and Erk signalling are needed to exert this function. This work has further delineated the specific signals controlling proliferation downstream of β1-integrin, and has provided a model to test the dependence of oncogenes for β1-integrin to drive proliferation in MECs. These studies are important to understand the role of β1-integrin in breast cancer formation and to define the types of breast cancer where β1-integrin can be used as an effective therapeutic target.