This Element focuses on phonetic and phonological development in multilinguals and presents a novel methodological approach to it within Complex Dynamic Systems Theory (CDST). We will show how the traditional conceptualisations of acquisition with a strong focus on linear, incremental development with a stable endpoint can be complemented by a view of language development as emergent, self-organised, context-dependent and highly variable across learners. We report on a longitudinal study involving 16 learners with L1 German, L2 English and L3 Polish. Over their ten months of learning Polish, the learners' perception and production of various speech sounds and phonological processes in all of their languages were investigated. Auditory and acoustic analyses were applied together with group and individual learner statistical analyses to trace the dynamic changes of their multilingual phonological system over time. We show how phonetic and phonological development is feature-dependent and inter-connected and how learning experience affects the process.