Vent blocking, the covering of the filter ventilation zone on a cigarette during smoking, is a potentially important aspect of smoking behavior. Various techniques have been used to assess the incidence of vent blocking, and widely different views have been expressed on its importance. Studies relevant to filter vent blocking have been reviewed with two overall objectives: to examine critically the evidence on the occurrence of vent blocking and to assess the effects of vent blocking on the smoke yield to the smoker. The reviewed studies fall into four main categories (1) measurements of the incidence of filter vent blocking among smokers; (2) the observed effects of vent blocking on cigarette ventilation and machine smoke yields; (3) the effect of experimentally blocking vents on human smoke yields; and (4) simultaneous determination of vent blocking and smoke yield under human smoking conditions. Direct observation indicates that only 4% of smokers have their fingers in direct contact with the cigarette during puffing. Estimates of vent blocking incidence by lips during smoking range from 15-24% (saliva-staining technique) to up to 50% ('tar' staining pattern technique) of smokers. For those smokers who do block the ventilation zone, a mean of 27% of the vents are blocked, and a maximum of about 50%.When the cigarettes are machine-smoked, the smoke yield increases in a highly non-linear manner as the blocked portion of the filter ventilation zone increases. This effect is also more pronounced at higher original filter ventilation levels. In contrast, smoking behavior monitoring techniques have shown that when the experimenter deliberately blocks the vent zone, the human smoker adjusts by taking smaller and fewer puffs. The blocked filter affects the yields of smoke components to the smoker less than it does smoking-machine measured yields. It is concluded that the incidence of vent zone blocking by fingers is quite low and relatively insignificant. The most reliable estimate for lip blocking is that up to 25% of smokers may cover the vent zone during at least one puff and for most smokers the coverage is partial. Ventilation zone blocking as it occurs in practice has only a relatively minor effect on human smoke yields compared to other smoker behavior factors. When a human smoker inadvertently partially or completely blocks the filter ventilation zone during smoking, he/she adjusts by taking smaller and fewer puffs. Because of these changes in puffing behavior during human smoking, predictions of the effects of filter vent blocking on smoke yields based solely on smoking machine yields are deceptive.