Breast cancer is a leading cause of global cancer-related deaths, and metastasis is the overwhelming culprit of poor patient prognosis. The most nefarious aspect of metastasis is dormancy, a prolonged period between primary tumor resection and relapse. Current therapies are insufficient at killing dormant cells; thus, they can remain quiescent in the body for decades until eventually undergoing a phenotypic switch, resulting in metastases that are more adaptable and more drug-resistant. Unfortunately, dormancy has few in vitro models, largely because lab-derived cell lines are highly proliferative. Existing models address tumor dormancy, not cellular dormancy, because tracking individual cells is technically challenging. This dissertation sought to develop a method that would enable the study of cellular dormancy of breast cancer in vitro. Here, I introduce a live cell lineage approach to find and track individual dormant cells, distinguishing them from proliferative and dying cells over multiple days. I applied this approach across a range of different in vitro microenvironments. My approach revealed that the proportion of cells that exhibited long-term quiescence was regulated by both cell-intrinsic and extrinsic factors, with the most dormant cells found in 3D collagen gels. I also fully characterized and applied existing three-dimensional bone marrow mimicking hydrogels to breast cancer studies, which can further be used to examine the role of the tumor microenvironment in regulating dormancy. When breast cancer cells were cultured in a serum-free environment in the bone marrow mimicking gels, they tend to form spheroids to proliferate rather than as single cells, compared to cells in serum-containing media. In addition, I examined the correlation between AXL and dormancy using the in vitro cell lineage tree approach and in vivo studies. Triple-negative breast cancer cells with low AXL expression showed a correlation with less detectable tumor growth in vivo. AXL knocked-out cell lines showed a larger dormant cell population, compared to the wild-type cells in vitro. I envision that this live cell lineage tracing approach will prove useful to biologists and bioengineers in the dormancy community to identify, quantify, and study dormant tumor cells.