Tumorigenesis is a dynamic process in with the mutant cells and their environment transform from a functional tissue into a dysfunctional assembly of cells. Interactions between the mutant cells and their neighboring cells, including the healthy surrounding tissue, the stromal cells, and the immune cells, are essential for mutant cells to expand and develop into a cancerous lesion. To understand how mutant cells are shaping their environment on the one hand, and how healty tissue dynamics dictate mutant cell behavior on the other hand, we follow the fate and dynamics of mutant mammary epithelial cells in their native normal tissue context using intravital microscopy.