In the past decades, there has been a surge in interest in the study of language complexity in second language (L2) research. In this article we provide an overview of current theoretical and methodological practices in L2 complexity research, while simultaneously framing these within the broader scientific interest into the notion of complexity. In addition to focusing on the role of complexity in L2 research, we trace how language complexity has figured in formal theoretical and typological linguistics. It is argued that L2 complexity research has often adopted a reductionist approach to the construct, both in terms of its definition and its operationalization. As such, previous L2 research has often confused related but conceptually distinct and operationally separable notions, such as relative and absolute complexity, and it has overemphasized syntactic and lexical forms of complexity at the expense of complexity related to morphology or linguistic interface phenomena. We then discuss a collection of five empirical studies which react to several of these issues by highlighting hitherto underexplored forms of complexity, adopting an explicitly cross-linguistic perspective or by proposing novel forms of L2 complexity measurement.