A number of physical factors that influence the relation between measured radar reflectivity and surface rainfall are considered both theoretically and through detailed comparisons of radar and raingauge measurements. These factors include natural differences in raindrop-size distributions, enhancement of radar reflectivity by presence of hailstones or melting snow, diminution of reflectivity by downdrafts, and low-level changes in rainfall rate caused by accretion or evaporation. Results of 374 comparisons in twenty storms, which cover a wide variety of synoptic situations and rainfall patterns, are presented. Magnitudes of the effects of the different factors are estimated, and storm types where they are likely to be significant are pointed out. Also, some ways of compensating for the observed effects are suggested.