Ultra-slim cigarettes have been available for twenty years, and in some regions, the popularity of ultra-slim cigarettes is growing dramatically. Based on a survey of brands from North America, Asia, and Europe, typical filter lengths for ultra-slim cigarettes fall in the range of 27 to 30 millimetres. Pressure drop targets for these filters are generally comparable to those for filters of conventional circumference. However, with a circumference near 17 millimetres, ultra-slim cigarettes have half the face area of cigarettes made at a conventional circumference. The small circumference of ultra-slims influences their performance significantly, requiring unique filter materials to maintain acceptable product characteristics. Pressure drop measurement protocols and smoke-testing regimes specify volumetric flow rates. By contrast, pressure drop and filtration efficiency depend fundamentally on the velocity of the air or smoke. Reducing the face area of a filter by half at constant volumetric flow rate doubles the linear velocity of the fluid. Without a compensating change in filter material, filter pressure drops, even at constant filter density, would also double. Typically, filter materials with larger fibres are employed to maintain acceptable filter pressure drops. Even with specialized filter materials, small changes in filter weight may cause relatively large changes in filter pressure drop. Ultra-slim filters are also unique in the relative contributions of viscous and inertial flow to filter pressure drop. Measurements over a broad range of flow rates show that the contribution of inertial flow to pressure drop in ultra-slim filters is more than twice that for filters of conventional circumference. An understanding of the factors influencing the performance of ultra-slim filters enables the cigarette manufacturer to make effective design and material selection choices for these products.