Today, nanofluids are referred to as a rather broad class of finely dispersed suspensions. Therefore, before describing and discussing their properties, it is necessary to strictly define what we mean by them. It turns out that there are various distinct dispersed fluids that are known by a common term of nanofluids. Nevertheless, their properties vary considerably, and it is necessary to strictly distinguish them in order to establish regularities within each class. This is important because a loose understanding of what should be attributed to nanofluids often leads to inadequate conclusions about their properties. Since this book describes not only experimental facts obtained over many years but also provides simulation data, the flows of nanofluids also need to be classified to determine adequate methods for their flow simulations. Thus the structure of the chapter is as follows. Section 1.1 defines nanofluids and classifies them, Sect. 1.2 describes the methods for creating nanofluids, and Sect. 1.3 discusses methods for their characterization and stabilization. Finally, since this book mainly deals with the creation of nanofluids based on ready produced nanopowders, the last section is devoted to a brief review of the creation of nanopowders of nanoparticles and carbon nanotubes (CNTs).