Because the field of bilingualism is still relatively new, studies in the linguistics, psycholinguistics, language development and neurolinguistics of bilingualism have often produced conflicting results. It will be argued in this paper that some of the difficulties encountered by researchers could have been lessened, if not avoided, had close attention been paid to methodological and conceptual issues. Among the issues covered are bilingual participants, language mode, stimuli, tasks as well as models of bilingual representation and processing. Each issue is dealt with in the following way: first it is explained, then the problems it causes are discussed, and, finally, tentative solutions are proposed. Examples are taken from descriptive and experimental studies of normal bilingual adults and children as well as bilinguals suffering from aphasia and dementia.