Based on results of a project undertaken in Finland over many years, this definition of transfer is offered: Transfer is both a facilitating and limiting factor which provides one basis for the learner to form and test hypotheses about the second language he or she is learning. Theory and research on transfer are discussed as they relate to item versus system learning processes, perceived language distance or relatedness, the learner's receptive versus productive competence, and the learner's stage of learning. It is concluded that the four most important variables in relation to the role played by the first language in learning the second language are: (1) the learner's age and mode of learning, with adults using transfer more than children and with more transfer occurring in a foreign-language learning situation than in a second-language learning situation; (2) spontaneity of utterance, with more transfer in elicited utterances, particularly when much time is available for response; (3) the learner's proficiency in the second and other languages; and (4) differences in individual learning styles. More quantitative research on the role of these factors is recommended. A three-page list of references concludes the study. (MSE)