This article discusses methodological issues related to language in advertising research. We introduce a framework that distinguishes between cross-linguistic research settings, where several languages are used in the study and different samples of respondents are studied in their own language, and multilingual research settings, where only a single language is used and multilingual respondents are studied either in their native or nonnative language. We review key principles that govern cross-linguistic and multilingual effects in advertising research to formulate guidelines for research design and data analysis. In the cross-linguistic context, these principles address nonuniform cross-linguistic differences in responses (related to nonequivalence of individual questionnaire items) versus uniform response effects (related to nonequivalence of verbal response category labels). In the multilingual context, we bring together evidence that shows how—even when comprehension is not a problem—stimuli, questions, and response categories may be processed differently in respondents' native versus nonnative language.