Informed by the perspective of complexity theory, this study seeks to explore two English-as-a-foreign-language (EFL) teacher educators' expertise in Hong Kong. Drawing on data from interviews, field observation, and informal communication over one academic year, the study revealed that the teacher educators' expertise entailed different components: (1) a dialogic teaching approach informed by research-based and contextualized knowledge; (2) emotional intelligence for maintaining personal emotional wellbeing and channeling emotions for effective teaching; (3) a reflective and agentive mindset for continuous learning; as well as (4) willingness and capacity to promote and distribute expertise in multiple communities. The participants' expertise functioned and transformed through their continuing engagement in different forms of teacher education practice, influenced by their personal beliefs and situated contexts. The study concludes with practical implications for language teacher educators on how to develop their expertise as a complex, adaptive, and evolving system in daily practice.