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Barrier/lagoon systems occupy a significant part of the world's coast. They are diverse in size, morphology, geological and oceanographic setting, and morphodynamic behaviour. Understanding the behaviour of barriers at 101 to 102 year and 101 to 102 km scales (mesoscale) is an important scientific and societal goal, not least because of the preponderance of intensive coastal development in a time of global climate change. Such understanding presents significant challenges. Challenges in describing mesoscale system behaviour relate largely to the incomplete evidence base of (i) morphological change in system components, (ii) dynamic and internal forcing factors (drivers) and (iii) geological constraints. These shortcomings curtail the development of baseline datasets against which to test models. Understanding observed changes and thereby predicting future behavioural patterns demands assumptions and simplifications regarding the linkages between initial state, dynamic drivers, system feedbacks and a multiplicity of geological constraints that are often location-specific. The record of mesoscale change is improving with the acquisition of long-term morphological datasets. Advances in technology and chronological control mean that geological investigations can now provide decadal to century-scale temporal resolution of morphological change. In addition, exploratory modelling is improving understanding of the influence of various dynamic and geological factors. Straightforward linking of dynamic forcing and response is seldom able to account for observed mesoscale behaviour. Geological factors exert a significant or even dominant control on barrier behaviour at decadal to century timescales. Whereas these geological controls can be quantified to some extent by detailed investigations of contemporary barrier/lagoon morphology and constituent materials, underlying geology and topography and sediment supply, in all but a few locations, such data are absent. This sets an unavoidable constraint on efforts to quantitatively predict the future behaviour of barrier systems, which are strongly site-specific in terms of their geological setting and morphology. Geological controls exist in a network of interactions that individually and collectively influence mesoscale barrier behaviour. Dominant, first-order controls are: • Basement slope; • Basement irregularity and erodibility; • External sediment supply; • Orientation; and • Shoreline lithification (beachrock and aeolianite) An important intermediate level of geological control is exerted by shoreface morphology. Shorefaces are themselves influenced by underlying geological factors, but they are dynamic at longer timescales than barriers. Geological influences are in most cases unquantified and are usually disregarded when conceptualizing and modelling barrier evolution. Consideration of the geological influences is, however, essential in efforts to predict future behaviour at mesoscale (management) timescales.