Lexical transfer is a complex and multifaceted phenomenon influenced by a number of factors (cf. Jarvis 2000), and which takes different forms and manifestations (cf. Jarvis 2009, Jarvis and Pavlenko 2008). Thus, we can  distinguish between positive, e.g. cognates, and negative transfer, which results in erroneous renderings; either at the formal level, e.g. borrowing, lexical creation or adaptation, or semantic and conceptual transfer such as calque or literal translation, or even transfer in lexical choice (cf. Jarvis 2000, Ringbom e.g. 2006). It can be the result of an unconscious process, a conscious compensatory strategy, or the by-product of applying an equivalence assumption. The present talk will offer a brief theoretical overview of lexical transfer to focus then on some of the variables that constrain or influence the phenomenon in SLA.  We will review the results of our own and other studies in which the relationship between lexical transfer and some intervening variables is explored. Specifically, we will talk about the role of the particular L1 background, age, L2 proficiency (frequently co-occurring with age), gender, and educational approach in lexical transfer episodes and will comment on the results obtained. The amount and type of lexical transfer has been found to be modelled by these variables. By looking into the products of the transfer, where visible, we can take insights into the process of transfer itself, and thus into how foreign language vocabulary is learned, stored, and accessed or retrieved.