This chapter investigates the acquisition of sentence-final particles (SFPs) by L2 and L3 Mandarin Chinese learners. It reveals that L2 learners can, with relative ease, acquire some SFPs (e.g. the yes–no question SFP ma) due to their high saliency and typological contrast with learners’ L1. However, learners at different proficiency levels demonstrate varying degrees of difficulty in acquiring other types of SFPs, which are attributed to degrees of the transparency and detectability of relevant cues in the input. The chapter further examines features attached to de in L2 Chinese shi …de constructions, revealing that the feature attachment to the SFP de develops in an uneven manner in English speakers’ L2 Mandarin, with the greatest challenges being those features involving cross-domain feature reassembly and those requiring sensitivity to pragmatically subtle cues in the input. The chapter also investigates ways in which SFPs in L3 Mandarin interact in learners’ minds with previously acquired languages (e.g. English and Cantonese), showing that the existence of an SFP counterpart in a previously acquired language does not necessarily facilitate L3 acquisition of the feature attached to the SFP and that transfer from a previously acquired language can be detrimental as well as facilitative.