Bone marrow chimeras are used routinely in immunology research as well as in other fields of biology. Here, we provide a concise state-of-the-art review about the types of chimerisms that can be achieved and the type of information that each model generates. We include separate sections for caveats and future developments. We provide examples from the literature in which different types of chimerism were employed to answer specific questions. While simple bone marrow chimeras allow to dissect the role of genes in distinct cell populations such as the hematopoietic cells versus non-hematopoietic cells, mixed bone marrow chimeras can provide detailed information about hematopoietic cell types and the intrinsic and extrinsic roles of individual genes. The advantages and caveats of bone marrow chimerism for the study of microglia are addressed, as well as alternatives to irradiation that minimize blood-brain-barrier disruption. Elementary principles are introduced and their potential is exemplified through summarizing recent studies.