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Wet and rainy conditions impact driver, vehicle, and roadway. These effects cause a reduction in speed or density (car-following headway) or both which, in turn, cause a reduction in highway capacity. This paper is a synthesis of 26 studies relating wet conditions to speed and capacity. If the results from all reviewed studies after 1980 with original data from freeways are averaged assuming equal weights, then, the average speed reduction is 4.7 mph in light rain (11 studies) HCM2000 suggests 6.0 mph which is similar to FHWAs 10 % reduction and 19.6 mph in heavy rain (2 studies) HCM2000 suggests 12.0 mph which is higher than FHWAs 16% reduction. The average capacity reduction is 8.4 % in light rain (7 studies) and 20.0 % in heavy rain (1 study). The impact of rainy conditions on highway capacity is important. Much additional research is needed for reducing the wide variance of observations in past studies. A methodology is needed for capacity and LOS analysis of freeways, intersections and arterials that accounts for wet conditions as part of typical conditions.