This chapter introduces key terminology and concepts necessary to embark upon a research project. It aims to sample research areas that fall under the umbrella of bilingualism and illustrates how methodological differences and limitations can influence findings. The chapter considers specific methodological aspects in conducting a study with bilinguals. An example of experimental research with bilinguals may consist of asking bilinguals to label pictures in either their first or their second language and comparing reaction times in the picture naming task across the two languages. In the example considering language therapy for bilingual children with language impairment, performance on a language assessment scale is the dependent variable. Studies of bilingual language processing usually focus on activation of the two languages, examining variables that influence the levels of activation of each language at each processing level, and the interactions within and between the two languages.