The diffuse irradiance on an inclined surface is usually estimated from the hourly horizontal irradiance measurements with a slope irradiance model. It is also possible to calculate the slope irradiance by integrating the sky radiance distribution generated with a sky radiance model. In this paper, five slope irradiance models and six sky distribution models are compared with the hourly irradiance measurements on 24 inclined surfaces in Turku, Finland (60 degrees 27＇N, 22 degrees 18＇E). Of the sky distribution models, the Perez all-weather sky model agrees best with the measurements. Of the slope irradiance models, the Reindl model has the lowest average mean bias difference (MBD), but the Perez slope irradiance model gives the lowest root mean square difference (RMSD) for all but one of the 24 surface orientations. The average RMSD for the Perez all-weather sky model is 1.5 percentage points lower than for the Perez slope irradiance model.