The success of internal fertilization is reliant upon successful sperm migration through the female tract. Timely location of the oocyte in what is a complex three-dimensional, highly invaginated series of moist opposed surfaces is a challenge at which only tens of sperm ever succeed. In part this could be due to the differences in scale, with a 50 µm long cell facing a probable migration of well over 20 cm due to the complex architecture. Many groups have focused upon the role for a chemotactic 'attractive egg' effect in guiding sperm to increase numbers at the fertilization site. What most research has neglected to consider is the role that the viscosity of the mucous layers, which coat the entire tract and through which sperm must swim, plays in both sperm selection and ongoing modulation of their behaviour. From allowing sperm to enter through the cervix during the ovulation phase, to denying them entrance through action of the female contraceptive pill, viscous effects are fundamental in controlling the migrating sperm population throughout the tract. The physiological effects of viscosity are also crucial to consider when designing and extrapolating data from in vitro experiments to the in vivo situation.