Microfluidics has been an active research field and an indispensable tool for quantitative analyses of chemicals and biological specimens for decades. Its widespread application stems from its ability to accurately handle small quantities of samples and its well-controlled environments. Further developments of microfluidics require a fundamental understanding of fluid dynamics and associated phenomena. This chapter provides some basic descriptions of fluid properties and principles that govern the flows inside the channels of a microfluidic device. It also examines relevant dimensionless numbers used for the understanding of the flow and the design and scale-up of microfluidic devices. The chapter concludes with suggestions on key areas of importance for future research in microfluidics.