Abstract Scale inhibitors (SI) have been applied very successfully over many years in oilfields to prevent the formation of mineral scale. Both barium sulphate and calcium carbonate scales may be prevented using inhibitors, although in this work we will focus on the more difficult barite inhibition problem. A number of publications have appeared discussing the mechanisms by which barium sulphate scale inhibitors operate to prevent or retard scale formation. The mechanisms are discussed here in terms of (a) nucleation inhibition where the scale proto-crystals forms but are then disrupted or redissolved by the action of the inhibitor molecules, and (b) crystal growth inhibition where the inhibitor is thought to adsorb or interact with the active crystal growth sites (growing edges or spirals) hence retarding or stopping the crystal growth process. Both of these mechanisms are consistent with the inhibition of mineral scale at "threshold" levels, typically for MIC = 0.5 – 20 ppm (MIC = minimum inhibitor concentration for a defined level of inhibition for a given test procedure). The MIC is always considerably below stoichiometric values in terms of the scale inhibitor to mineral scale molar ratios. It is known that most inhibitor types from the small molecular phosphonates (e.g. DETPMP) to polymeric species (e.g. PAA, PVS, PPCA) actually operate through both of the above mechanisms although one of these may predominate for specific species. Previous work has established that, broadly speaking, smaller phosphonates operate principally as crystal growth inhibitors and polymeric species work mainly as nucleation inhibitors. A number of factors are known to affect the inhibition efficiency (IE) of scale inhibitors against barite formation, the main ones of concern here being pH, temperature and the calcium and magnesium levels in the scaling brine mixture. The general observed effects of these factors have been described in the literature and will be discussed in detail in this paper. However, no complete description of the mechanism of barite inhibition has appeared which clearly and consistently explains all of the observed effects of these parameters for different scale inhibitor types. It is the central aim of this paper to present a complete and consistent set of mechanisms for barite inhibition which may vary in degree for different inhibitor types. Our proposed mechanisms are based on a wide range of observations from the open literature analysed with our own experimental and modelling results.