This chapter summarizes the practices applied and challenges experienced by the World Values Survey (WVS) in the survey harmonization procedures. Since its emergence back in 1981 in partnership with the European Values Study, the WVS's initial goal was to produce comparable cross-country survey data. Due to this aim, input and ex-ante output harmonization have prevailed as the main data harmonization paradigm throughout the whole WVS's history. The main subject of harmonization in the WVS remains its measurement instrument – the WVS master survey questionnaire. Input harmonization is applied to all core and majority of demographic and socioeconomic variables, including income and occupation group; ex-ante output harmonization methods are used for country-specific variables such as religious and ethnic belonging, education level, and spoken language. Substantial harmonization efforts in the most recent WVS survey round (2017–2021) are dedicated to the utilization of the ISCED-2011 international classification of education levels, ISO 639-1 (alpha-2 and alpha-3) list of language codes, and a new common target variable for the respondents' religious belonging. To further improve data comparability, the WVS has pursued an input harmonization of sampling and survey methodology by requiring all national surveys to follow common provisions on the sample design and interview procedures. Pre-survey validation was introduced to ensure compliance with the official questionnaire and sampling rules. Data harmonization in the WVS has been powered by its own software “JDSurvey” developed by the WVS Data archive.