The growing interest in using zebrafish for genetic and functional dissection of malignancy and infection was highlighted by the second international workshop on Zebrafish Models of Cancer and the Immune Response in Spoleto, Italy (July 20-22, 2009). The overarching theme of the state-of-the-art reports featured the unique suitability of zebrafish for in vivo monitoring of fundamental biologic and pathologic processes. For example, in vivo imaging was employed for the first demonstration of direct development of hematopoietic stem cells from hemogenic epithelium and for visualization of T-cell homing and interaction with thymic epithelial cells. In addition, in vivo monitoring was instrumental for developing disease models of solid tumors, leukemia, and of inflammatory conditions, and for assessing the efficacy of small molecule drugs under physiologic and pathologic conditions. The success of zebrafish small molecule screens was underscored by the identification of prostaglandin E2 (PGE2) as an efficient inducer of stem cell expansion that led to the initiation of the first human trial on the efficacy of PGE2 in bone marrow transplantation. Further, zebrafish models of infectious diseases such as tuberculosis have been established that are now amenable to high-throughput in vivo drug screens, a much-needed development in the fight against drug-resistant microorganisms. The success of this workshop and the rapidly growing field of cancer and the immune response in zebrafish have spawned follow-up meetings in Boston (June 2010) and Edinburgh (2011).