Abstract In this review we discuss how viscosity contrasts between the injection plug and the mobile phase may lead to loss in separation performance, especially in UHPLC columns or SFC environments. Firstly, the wall effect is discussed, and how it can amplify viscosity contrast effects. We then illustrate how viscosity contrasts lead to the phenomenon known as viscous fingering, and we detail the destructive effects of this phenomenon. We expand on the viscous fingering component, however, demonstrating that viscosity contrast effects begin to deteriorate performance long before the conditions are such that viscous fingering occurs. Subtle changes in band-shape are apparent even with very low viscosity contrasts. Lastly we illustrate how viscosity contrast effects lead to severe peak distortions in SFC. Analysts who seek high efficiency separations must make every effort to eliminate, or at least minimise the viscosity contrast between the injection plug and the mobile phase.