During last decades, post-earthquake damage (e.g., Kocaeli 1999, L'Aquila 2009, Lorca 2011) numerical and experimental studies showed that a growing attention should be addressed to the influence of infills on the seismic performance of Reinforced Concrete (RC) buildings. In this paper, a numerical investigation on the influence of infills on the seismic behavior of RC structures is carried out. Four different case study buildings are analyzed: four- and eight- storey buildings, designed for seismic loads according to the current Italian technical code (D.M. 2008) or for gravity loads only according to an obsolete technical code. Seismic capacity at two Limit States (Damage Limitation and Near Collapse) is assessed through static push-over analyses, within the N2 spectral assessment framework, and different infill configurations are considered (Bare, Uniformly Infilled, Pilotis). A sensitivity analysis is carried out, in order to evaluate the influence of main material and capacity parameters on seismic response, depending on the number of storeys, the design typology and the infill configuration. Fragility curves at both Limit States are obtained, through the application of a Response Surface Method. Seismic performance is also expressed in terms of failure probability, given a reference time period. Moreover, a comparison between fragility analysis results and observed damage during L'Aquila earthquake is carried out, in particular at Damage Limitation Limit State.