Experimental data for convective heat transfer from horizontal cylinders (diameter approximately 1 2 in.) to water in crossflow have been obtained. It is shown that these data can be relegated to four zones, depending upon the value of the ratio G f R f −2, where G f and R f are the Grashof and Reynolds numbers, respectively, with properties evaluated at the mean film temperature. In the first zone, defined by G f R f −2 < 0.5, the predominant heat transfer mechanism is forced convection, and previously published forced convection equations may be applied with negligible error (less than about 5 per cent). In the second zone, defined by 0.5 < G f R f −2 < 2, the predominant heat transfer mechanism is still forced convection, but here natural convection affects the overall heat transfer coefficient by as much as 10 per cent and this is not considered to be negligible. Empirical correlation equations are presented by means of which it is possible to account for natural convection in zone two for three directions of forced flow: vertically upward, horizontal, and vertically downward. Zone three, defined by 2 < G f R f −2 < 40, is a region in which natural and forced convection effects are of the same order of magnitude. In a subregion of this zone (4 < G f R f −2 < 40) the heat transfer process is unsteady when the direction of the forced flow is horizontal; under such conditions the heat transfer coefficient oscillates with random period between two extreme values. It is hypothesized that this oscillation is a result of periodic changes in the boundary layer flow—the lower extreme value of the heat transfer coefficient is believed to correspond to the periodic establishment of laminar boundary layer flow resembling the flow associated with pure natural convection, and the higher extreme value is believed to correspond to the periodic establishment of the kind of separated boundary layer flow normally encountered near cylinders in forced crossflow. Zone four, defined by G f R f −2 > 40, is a region in which natural convection effects predominate. A simple theory has been developed to describe the behavior of the heat transfer coefficient in zone four. A fifth zone is identified which contains none of the data gathered here.