ABSTRACT: Orvieto rises on top of a mesa formed by a slab of weak pyroclastic rocks overlying an overconsolidated clay formation. The particular geotechnical conditions of the site enhanced the erosive process that isolated the mesa and in turn favoured human settlement. These geotechnical conditions include the stiffness contrast between the slab and the clay, the low strength of the pyroclastics, and the susceptibility to degradation of the clay formation. These conditions have been found to be the source of a complex and time-dependent interaction between landslides and urbanization. In this paper the anthropic changes to the natural evolution of the landscape are reconstructed through archival and historic documents over the last eight centuries and interpreted from a geotechnical point of view. The types and characteristics of failures that occurred in the past are described by means of aerial photo analysis, field surveys and archival research, whilst geometry and kinematics of active movements are illustrated using present-day monitoring results. Based on a detailed interpretation of displacement and pore pressure monitoring in the clay slope and the results of numerical analysis of the erosional process and selected failures, a conceptual model of the instability mechanism of the hill is proposed. Finally, protection policies from the Renaissance period until the XX century, when comprehensive preservation projects started, are illustrated and their impact on the instability mechanism discussed.