This book explores simultaneous trilingual siblings' code switching behavior focusing on their minority language development combined with a thorough examination of their minority language speaking parent's input. It explores their complex discourse in relationship to the key input, the minority language speaking parent's responses to the children's mixed utterances. This social-interactional and context-based perspective approach sheds new light on the fascinating facet of children's language manipulating ability. The book describes how the siblings build up a systematic method of inserting non-Korean words into the Korean language structure and argues code switching is an organic building block in a child's multilingual system and provides a picture of its developmental process.