Primary tumors shed circulating tumor cells (CTCs) into the bloodstream that metastasize preferentially to distant organs, resulting in 90% of cancer related fatalities. For example, estrogen receptor positive (ER+) breast cancers exhibit high rates of metastasis to bone, with decreased rates to liver and lung. CTCs exhibit heterogeneous gene expression programs and functional phenotypes, which are selected by soluble and mechanical interactions within each metastatic “niche.” A critical challenge is to predict how patient-specific CTCs disseminate throughout the body and respond to therapeutic treatments. An exciting strategy is to culture CTCs ex vivo for drug screening informed by genomic and transcriptional profiling. We seek to elucidate how CTCs respond to different features of the metastatic niche by engineering controlled interactions with tissue specific extracellular matrix (ECM) and with human primary stromal cells, which may recapitulate disease progression and therapeutic resistance in these microenvironmental contexts.