This course investigates Complexity Theory (CT) as a lens to study Second Language Learning and Development (SLLD). After presenting the origin, evolution and main tenets of the constellation of complexity in a transdisciplinary perspective, the course turns to investigating key concepts such as emergence, complex adaptive systems, fractals, change over time, and attractors in relation to SLLD. CT is increasingly used as a metatheory in the study of phenomena related to the development of languages, the positioning of language learners/users vis-à-vis their own linguistic trajectories and repertoires, and the conceptualization of language learning. The course studies phenomena which imply overcoming linguistic/cultural barriers and triggering individuals’ creativity and agency, as well as the way CT can be articulated to existing theories of (language) learning (specifically Vygotsky’s Sociocultural theory and Bandura’s Social cognitive theory) and studies in the field of creativity. Students will identify areas of SLLD that would benefit from a CT-informed framework, will discuss the implications of using it, alone or in combination with other theories, and will analyze research studies that have adopted CT as their theoretical lens.