Resumen
This paper discusses some key points related to the ontology of the ?appropriate assessment? [1] procedure concerning plans significantly affecting Natura 2000 sites. We study this ontology by discussing its implementation into the adjustment process of the master plans of the regional municipalities of Sardinia (Italy) to the Regional Landscape Plan (RLP) and put as evidence some important general observations, coming from the case study, concerning the utility and effectiveness of the ontological conceptual framework in order to help planners and decision-makers understand and structure the assessment process of plans.