The main goal of the project is the analysis of coverage error for the implementation of web surveys aiming at the general population in the Spanish context. When surveying the general population through the Internet, coverage bias appears as the first problem as not everybody has Internet access. Internet penetration rates in Spain still stand between 40%-70% of the population depending on the sources (Eurostat, Internet World Stats, INE, EGM, Tatum), and the criteria used to measure Internet coverage (Household access, type of connection, last time used -year, three months, yesterday-, frequency of use, etc). In order to gradually introduce web surveys as a valid and legitimized mode of data collection for surveying the general population in the CIS studies, the present project analyses the determinants of coverage error in Spain for each of the measures of Internet coverage available at the CIS database. The initial hypothesis is that Internet and not Internet users differ not only regarding their socio-demographic characteristics, but also in terms of political/ideological and attitudinal or webographics variables. The knowledge of these determinants could be of great help when implementing web surveys, for instance in the design of online panels or using mixed-modes of data collection. The study is conceived as the first step of a long-run process for the inclusion of web surveys in official statistics and research institutes, focusing on the Spanish case and the CIS.