Partial factors have emerged witin limit state design methodology as a means of adjusting parameters so that appropriate values are derived for design calculations. They have become associated with uncertainty, but their proper use in geotechnical engineering is to safeguard against predictable deformation by factoring down the peak soil strength. Ultimate soil strengths must be used to check for safety against collapse: these parameters are easily and reliably measured and require no partial factor. Considerable effort must be expended to explain to structural engineers the real nature of serviceability and collapse limit state brought about by soil behaviour. A simple, unified approach is suggested which is based on the theory of plasticity.