This chapter concerns the physical and flow properties of fluids such as blood and other liquids. The field of rheology is introduced along with the definition of viscosity and the laminar flow of a fluid. This leads to the Hagen–Poiseuille equation which is used to describe the laminar flow of a Newtonian fluid. The rheology of blood is also covered and the Casson equation is used to describe the shear stress and shear rate relationship for blood. The Fahraeus and Fahraeus–Lindqvist effects are discussed and used to explain how the diameter of the tube can affect the observed or apparent viscosity of blood at high shear rates. Laminar boundary flow over a flat plate is also described. For turbulent flow, the Bernoulli equation is developed with the use of the friction factor to account for pressure losses due to the length of the tube and various fittings. The chapter concludes with a discussion on surface tension and the dynamics of capillary flow.