Simple SummaryA bottleneck in oncology is the translation of results from preclinical models to the clinics. The rate of anticancer drugs that are effective in preclinical studies but fail in clinical trials is more than 95%. In order to test new immunotherapies and to identify the most effective combination of anticancer drugs, next-generation mouse models have been developed. These “humanized mouse models” support the growth of patient-derived tumors and the development of a human immune system. This review provides an overview of next-generation humanized mouse models and how they can be used to advance precision cancer medicine and immuno-oncology clinical trial design.AbstractCancer immunotherapy has brought significant clinical benefits to numerous patients with malignant disease. However, only a fraction of patients experiences complete and durable responses to currently available immunotherapies. This highlights the need for more effective immunotherapies, combination treatments and predictive biomarkers. The molecular properties of a tumor, intratumor heterogeneity and the tumor immune microenvironment decisively shape tumor evolution, metastasis and therapy resistance and are therefore key targets for precision cancer medicine. Humanized mice that support the engraftment of patient-derived tumors and recapitulate the human tumor immune microenvironment of patients represent a promising preclinical model to address fundamental questions in precision immuno-oncology and cancer immunotherapy. In this review, we provide an overview of next-generation humanized mouse models suitable for the establishment and study of patient-derived tumors. Furthermore, we discuss the opportunities and challenges of modeling the tumor immune microenvironment and testing a variety of immunotherapeutic approaches using human immune system mouse models.