Due to the emerging applications in micro/nano-electromechanical systems (MEMS/NEMS), knowledge of fluid mechanics and heat transfer behavior at nanoscale dimensions has advanced significantly over the last few decades. MEMS/NEMS refers to small devices that combine mechanical and electrical components with feature sizes ranging from a few nanometers to hundreds of microns. This chapter introduces the micro- and nano-flows, and their applications will be described. Basic flow concepts related to micro- and nano-flows will be reviewed and discussed. The flow features related to such flows will be presented. The main features are slip and jump conditions. The discussion will focus on rarefied gas flows, which occur in gas flows at micro- and nano-scales. Different concepts related to dilute gases, rarefied flow, and the Knudsen number will be discussed. Following that, the gas flow classification based on the Knudsen number is introduced. In the final section of this chapter, unique flow phenomena of rarefied gas flows will be discussed and reviewed. They are cold-to-hot heat transfer, Knudsen minimum, nonlinear pressure distribution, Poiseuille number deviation, and thermal polarization.