Experimental mouse chimeras have served as immensely important research tools for studying many aspects of mammalian development ever since they first were produced over 50. years ago. Chimera studies have served as crucial assays in the era of modern mouse genetics that was triggered by the advent of mouse embryonic stem cells. Lately, chimeras are also used as proof of pluripotency and normality of induced pluripotent stem cells. With this long history in mind, it may seem surprising that chimeras now have an ever-increasing role to play. The high-throughput mouse gene targeting projects are in the process of producing ES cell lines with a mutation in each of the close to 20,000 known protein coding genes. These will all be waiting for germline transmission through chimeras. Such a large-scale approach calls for simplified methods for generating germline transmitting chimeras. In this chapter, we will describe the currently most cost efficient and simple method; the aggregation of pluripotent stem cells with diploid or tetraploid mouse embryos. Since most of the large knockout projects are using the C57BL/6 background, we will pay special attention to cell lines derived from this inbred strain.