Mouse models of cancer have provided much necessary insight regarding the processes of oncogenic transformation and tumor development over the last decades. However, few studies have investigated whether the core tumor-immune interactions that regulate cancer development and spread are conserved in mice, and yet accumulating evidence supports a central role for the immune system in impacting all facets of tumorigenesis and progression. Unfortunately, the complexity of the immune system and its behavioral states has surpassed the technical limitations of fluorescence-based flow cytometry and conventional immunohistochemistry (IHC). The development of mass cytometry and multiplexed ion beam imaging (MIBI) allows investigators to assay the immune system as a whole more thoroughly than ever before. Mass cytometry has expanded the number of parameters that can be simultaneously measured on single cells by combining the throughput of flow cytometry with the precision of mass spectrometry. Similarly, MIBI has dramatically increased the number of parameters that can be imaged to dozens of proteins in a single tissue section. The combination of these methods with new analytical algorithms enables a systems-wide comparison of immune organization between mice and humans. We will apply these novel tools to the setting of breast cancer with the specific intent of discerning similarities and differences between mouse models and primary patient samples of the same histological origin. Our specific aims are to 1) reveal similarities in the phenotype and function of immune infiltratesin metastatic versus non-metastatic breast cancer in humans and mice; 2) elucidate the composition and behavior of the immune system in humans and mice through progression from normal tissue through early neoplasia and metastatic disease; and 3) reveal the effect(s) of standard-of-care therapies on the immune infiltrate in mouse and human breast cancer. These studies will result in guidelines for the investigation of the tumor-immune network in pre-clinicalmouse models of breast cancer to determine which findings are likely applicable across the species barrier. Moreover, the proposed studies will lay the foundation for determining the suitability of mouse models in any cancer or for any treatment.