This chapter provides an introduction of Mark–Houwink–Staudinger–Sakurada (MHSS) constants. The viscosity of a dilute polymer solution depends on the nature of polymer and solvent, the concentration of the polymer, its average molecular mass and molecular mass distribution, the temperature, and the shear rate. The most important characteristic quantity in a very dilute solution, at vanishing shear rate, is a limiting viscosity number [η]. The limiting viscosity number for a series of homologous polymers under a fixed solvent condition (solvent species and temperature) follows the MHSS relation. In the MHSS equation both an MHSS exponent (a) and a coefficient (K) depend on the polymer solvent pair and the temperature. The parameters K and a are evaluated from the intercept and slope, respectively, of the double-logarithmic plots of [η] against Mv determined for a series of samples that differ only in their molar mass. The ratio of [η]/[η]θ is called the viscosity expansion factor α3. The viscosity-averaged molar mass obtained from the MHSS equation may also be used to get information about the degree of molar mass dispersion from the comparison of the values of a polymer sample in two different solvents. The MHSS constants are obtained for the [η]–M relationships expressed in terms of number-average molar mass or weight-average molar mass. Two extensive tables of K and a values of commonly used polymer–solvent systems exist.