A broad interest exists in developing structure-based indicators to use as proxies for other attributes that are difficult to assess, such as biological diversity. Summary variables that account for stand-scale forest structural complexity could facilitate the comparison among stands and provide a means of ranking stands in terms of their potential contribution to biodiversity. We developed an index of structural heterogeneity (SHI) for beech forests in southern Italy: (i) we established a preliminary list of 23 structural variables obtained from data routinely collected in forest inventories; (ii) we quantified these variables in a set of 64 beech-dominated stands encompassing a wide range of variability in the Cilento, Vallo di Diano and Alburni National Park; (iii) we identified a core set of attributes that take into account the main sources of structural heterogeneity identified in reference old-growth forests; and (iv) we combined these core attributes into a simple additive index (SHI). We identified eight core attributes that were rescaled to the range 0 to 10 using regression equations based on raw attribute data. The SHI was calculated as the sum of these attribute scores and then expressed as a percentage. The index performance was evaluated against ten reference old-growth beech stands in the Apennines. The index ranged between 38 and 79.1 (median=59.4) and was distributed normally for the calibration dataset. The SHI successfully discriminated between old-growth (range=71.9-99.9, median=85.1) and early-mature to mature forests. Furthermore, the SHI linearly increased with stand age and was higher in multi-layer high forests than in single- and double-layer forests. However, a large variation was detected within both management types and age classes. SHI could be helpful for foresters as a tool for quantifying and comparing structural heterogeneity before and after a silvicultural intervention aimed at restoring the structural complexity in second-growth stands.