Viscous fingering (VF) is an important fluid transport phenomenon that manifests itself when two fluids having different viscosities move in the same direction. Their interface is unstable and a complex fingering pattern may arise. The complex pattern of VF is chaotic. Although its evolution on the short time-scale is somewhat predictable, on a long time-scale it is not and the transition is complex, evolving slowly at very low Reynolds numbers (Re < 0.005). At higher flow velocities, the chaotic effects arise earlier, become more intense, and evolve faster. The VF phenomenon may be important in chromatography because it could lead to a decrease or even a failure in separations. The onset of viscous fingering was visually observed by packing a glass column with particles having the same refractive index as the mobile phase and injecting plugs of dye solutions having viscosities different from that of the mobile phase. Severe fingering effects are observed if the viscosity contrast exceeds 0.17 cP. For smaller viscosity contrasts, band distortions are observed that affect to a degree retention data, band efficiency, and band resolution. Careful attention should be paid to matching the mobile phase viscosity and that of the injection plug when accurate chromatographic information is required.