The ultimate capacity of masonry structures is affected by several factors, such as geometry, external actions, material properties, and environmental conditions. The accurate definition of the mechanical properties is quite challenging due to various sources of uncertainties arising from workmanship techniques, block shape and texture, properties of single components, environmental conditions, existing damage, deterioration, and other factors. A major drawback is the lack of sufficient experimental data for the accurate probabilistic characterization of the uncertain properties. In this Chapter, uncertainty analysis of masonry structures is addressed within a non-probabilistic framework using the interval model, which is suitable to describe uncertainties when only range information is available. To limit overestimation affecting interval computations, the uncertain parameters are described by means of the Improved Interval Analysis via Extra Unitary Interval. As a practical application, the bounds of the in-plane resistance of unreinforced masonry walls involving interval uncertainties are estimated by interval extension of the analytical formulations provided by the Italian Building Code.