The orientation of subtidal hard substrata has long been known to play a key role in the assemblage that develops and the assumption has been that the nature of the substratum plays a minor role in this regard. I examine the features of mineral, living and artificial surfaces that affect the development of sessile invertebrate assemblages and find evidence that not all hard substrata are equivalent. As an example, evidence is mounting that rock type can affect the structural complexity of the reef at a range of scales: from millimetres, due to mineralogical dissolution, to the km scale. Furthermore and rather surprisingly, mineralogical content may be an important structuring agent. A further exciting development in this field is the finding that surface microtexture provides antifouling defence for a broad range of taxa. As ecologists working on subtidal assemblages, the challenge facing us is to understand processes that underpin pattern in these systems, particularly on natural surfaces and at depths beyond SCUBA.