Understanding the hydrological behaviour of urban surfaces is imperative in the design of surface water drainage systems and flood mitigation strategies, as well as for the modelling of groundwater recharge and pollution. This study has examined the hydrological behaviour of cracked impervious surfaces through field infiltration testing and image analysis of the cracks themselves. Infiltration tests were undertaken on a section of concrete slab pavers paving. Our results showed that cracks in impervious surfaces allow significant volumes of water to infiltrate through them, with infiltration rates comparable to those found in sands and gravels. Using a regression model, infiltration rates were related directly to crack characteristics obtained from image processing software, thereby enabling the first published quantitative link between percentage cracked area and infiltration capacity. The implications of accounting for this infiltration for surface water management systems are estimated to be in the order of £20 million annually for the construction industry in England.