Breast cancer is the most common cancer to affect women and can be grouped into six molecular subtypes. Although our molecular understanding of breast cancer has made rapid progress, the 3-dimensional structure of breast tumours and the interactions between individual tumour cells are not well understood. We have previously developed a state of the art 3-dimensional imaging technique that we used to reveal the role of bipotent stem cells in the mammary gland. We are now applying this technique to study tumourigenesis in the mammary gland and are moving into the fourth dimension of time; performing intravital microscopy to understand the behaviour of breast epithelial cells and how this changes in cancer and metastasis. To develop 3D and 4D imaging in tumourigenesis we are utilising the MCF-7 and MDA-MB-231 human breast cancer cell lines. We are generating cell lines expressing reporter genes of different colours, mixing these and transplanting them into immunocompromised mice. The resulting multicoloured tumours are being analysed by 3D confocal imaging on scales of up to 2 cm at cellular resolution. Despite the huge scale of these images we are able to investigate the interaction of tumour cells with the vasculature, immune cells and fibroblasts at high resolution. These images have shown that large areas of tumours become dominated by single colours over time. Two-photon imaging of our multicoloured tumours will allow us to observe the behaviour of individual tumour cells over times of up to 8 hours. Here we discuss our current progress in applying 3 and 4-demensional imaging to study and understand breast tumour structure and how tumour cells interact within this environment.