Wettability measurement methods, the effect of wettability on fluid distribution, and fluid flow in porous media were discussed, and the influence of rock wettability on the relative permeability and recovery of oil by waterflooding were investigated. Experimental studies were conducted on a total of 23 core plugs from two different limestone formations. Synthetic brine (NaCl solution) and mineral oil, which has a viscosity ratio of similar to10, were used as the test fluids. Core samples, saturated with synthetic brine, were flushed with mineral oil to establish the initial conditions, and the wettability of the samples was measured by the Amott-Harvey method. Test results showed that the wettability of the samples ranged from strongly water-wet to intermediately wet. Wettability measurements were repeated and showed that the Amott-Harvey wettability indices were reproducible. The effects of aging time, brine salinity, and saturation procedure on wettability were examined. Long-time aging in mineral oil altered the wettability of a water-wet core sample to intermediately wet. Unfortunately, there was only one water-wet sample, and correlation of the alteration of wettability, with respect to aging time, was not possible. Increasing the brine salinity also reduced the water wetness. However, the saturation procedure had almost no effect on the wettability of the core samples. Aging could not alter the wettability of the intermediately wet samples, by varying brine salinity and by changing saturation procedure; therefore, it was concluded that intermediate wettability was more stable than water wetness. Relative permeability and waterflood studies were conducted on core samples that had different wettability levels. In a water-wet medium, water breakthrough occurred relatively late and a considerable amount of oil was produced before the breakthrough. After the breakthrough, the rate of oil production was decreased very sharply. Decreasing the water wetness resulted in decreasing the breakthrough recovery. In the intermediately wet system, breakthrough occurred very early and most of the oil was produced after the breakthrough. Water breakthrough had almost no effect on the rate of oil production. Waterflood in the water-wet system seemed more economical, because a lesser volume of water was required to produce the same amount of oil.