The compressibility and strength characteristics of reconstituted clays are used as a basic frame of reference for interpreting the corresponding characteristics of natural sedimentary clays. The properties of reconstituted clays are termed ‘intrinsic’properties since they are inherent to the soil and independent of the natural state. The properties of a natural clay differ from its intrinsic properties due to the influence of soil structure (fabric and bonding). Thus the intrinsic properties provide a frame of reference for assessing the in situ state of a natural clay and the influence of structure on its in situ properties. A new normalizing parameter called the void index is introduced to aid in correlating the compression characteristics of various clays. The sedimentation compression curves for most, but not all, natural clays lie well above the corresponding intrinsic compression curves. A consequence of this is that such clays are more sensitive and brittle than the reconstituted material and the post-yield compression index Cc is usually much greater than the intrinsic value. This observation has important consequences for stress-path testing of soft clays. The location of the natural sedimentation curve relative to the intrinsic one is shown to depend on depositional conditions and on post-depositional processes such as leaching. The undrained strength of a normally consolidated natural sediment is shown to be primarily a function of the in situ effective stresses and of the soil structure and not of the moisture content. For overconsolidated natural clays the intrinsic compression line provides a useful means of assessing the degree of overconsolidation. Also the ratio of the intrinsic swelling index to the undisturbed swelling index (the swell sensitivity) is a valuable measure of bonding. The strength properties of two overconsolidated clays (Todi Clay and London Clay) are presented and the intact strengths are shown to be greater than the corresponding intrinsic strengths. However, both clays show brittle behaviour with the formation of shear surfaces at peak intact strength. The strength on such a shear surface drops rapidly to a well defined post-rupture strength after a few millimeters relative displacement The post-rupture strength must be clearly distinguished from the residual strength which requires much larger relative displacements to develop. Evidence is given which indicates that the post-rupture strength may be relevant to many stability problems in stiff clays and may also control the in situ stresses during geological unloading. For Todi Clay and London Clay the post-rupture strengths at low confining stresses are close to the intrinsic critical state strengths. More study is required before this can be accepted as a general result for most clays.