A review of the concept of a bulk viscosity coefficient β is presented, involving a discussion of theoretical approaches and a summary of existing experimental data. Two independent viscosity coefficients μ and A are obtained in the viscous stress tensor as a result of the isotropic Newtonian assumption. Hence, all solutions of the Navier-Stokes equations must make an assumption regarding the functional form of A. With β=λ+2/3μ by definition, the assumption is often arbitrarily made that the two viscosity coefficients are not independent, with β =0. This assumption has been shown to be valid only for dilute monatomic gases and is equivalent to stating that dilatational flowfield effects are not significant. Theoretical approaches for quantifying β are classified according to whether the subject fluid is dilute or dense. Experimental methods used to estimate bulk viscosity are described. Existing experimental data are summarized for each fluid classification, and issues related to using these data for dilatational flows in local thermodynamic equilibrium are addressed