Results of a wind tunnel experiment in which there are systematic variations of free stream turbulence above a flat plate boundary layer are presented. Upstream of the plate, an active grid generates free stream turbulence varying in intensity from 0.25% to 10.5%. The momentum thickness Reynolds number of the boundary layer varies from 550 to nearly 3,000. In all cases, the ratio of the free stream turbulence length scale to the boundary layer depth is greater than unity. Hot wire measurements show that, at high turbulence intensities, the effects of the free stream turbulence extend deep into the boundary layer, affecting the wall stress as well as the small-scale (derivative) statistics. Energy spectra show a double peak. At very low free stream turbulence intensities these peaks are associated with the inner and outer scales of the turbulent boundary layer, but at high turbulence intensities the free stream energy peak dominates over the boundary layer's outer scale. The results are contrasted with recent studies of turbulent boundary layers without free stream turbulence.