This article focuses on the research field of heat transfer of single-phase laminar flow and two-phase self-exciting oscillating flow in microchannels. First, to make prominent the special features of microthermal systems, the definition of the term "nanothermal systems" is discussed from the viewpoint of local equilibrium. Next, to show the special features of flow and heat transfer in microchannels, some thermal functions appearing in microchannels are introduced. Further, focusing on flow and heat transfer characteristics of single-phase laminar liquid-flow in microchannels, research is introduced that shows that the results obtained for tubes larger than 0.1 mm in inner diameter are in good agreement with the conventional analyses. Finally, Japanese research and recent results at IIS on Micro SEMOS heat pipes (M-SEMOSs) are introduced, and it is shown that a M-SEMOS of 0.5 mm in inner diameter can transport a significant amount of heat.