The paper deals with some theoretical aspects of language transfer as a psycholinguistic phenomenon. Firstly it presents the history of studies and theories on language transfer. Then, in its main part it describes the types of interpreting strategies caused by language transfer The description is based on the corpus of Polish texts interpreted into English and English texts interpreted into Polish by Polish interpreters native speakers of Polish). The analysis of transfer strategies is limited only to those which are classified in the retrospective reports (???) of the interpreters as transfer. In this way the analysis concerns more the awareness of transfer than transfer strategies proper since these are usually unconscious language production. The analysis is then interpreted in terms of the theory of language transfer.