Complex biological processes often require in vivo analysis. A fundamental understanding of many biological processes in humans has stemmed from experimental studies in animal models, particularly in laboratory mice. For several decades, our lab has studied the molecular and cellular basis for pathological changes caused by spontaneous mutations that disrupt the development or regulation of the murine hematopoietic and immune systems. This knowledge has increased our understanding of human disease. Certain mutations result in severe combined immunodeficiency disease (SCID). We have applied the knowledge gained in our studies of SCID mice to optimize them to serve as hosts for human normal and malignant cells and tissues. There is a growing need for animal models to carry out research studies without putting human individuals at risk. We have developed SCID mouse models that support high levels of engraftment with human cells and tissues to overcome these limitations. We have collaborated nationally and internationally with colleagues to develop improved humanized mouse models and optimize the technologies used for engraftment of normal and malignant human cells and tissues. Our research has leveraged these models for translational studies on human hematopoiesis, immunity, autoimmunity, infectious diseases, diabetes, regenerative medicine and cancer.