The interaction between bilingual phonetic systems is dynamic, shaped by both long-term and short-term factors. Short-term factors, the focus of this chapter, refer to immediate changes in the linguistic situation. This chapter discusses two short-term sources of potential phonetic interference: code-switching and bilingual language mode. A growing body of research on the phonetics of code-switching has shown that code-switching may result in phonetic interference between the L1 and the L2, although outcomes may vary across speakers and features. Language mode describes a bilingual’s position along a continuum, from operation in a monolingual mode, in which only one language is active, to bilingual mode with equal activation of both languages. Recent work has demonstrated that language mode may modulate cross-linguistic phonetic interference, with greater interference found during bilingual mode. Finally, this chapter discusses two variables responsible for modulating and constraining phonetic interference in cases of dual language activation that have emerged in the literature – the nature of the phonetic feature and bilingual language dominance.