Linguistic performance elicited by language tasks has generally been operationalized in terms of complexity,
accuracy, and fluency (CAF). However, this study argues that assessment of L2 proficiency is impossible without taking into
account the adequacy and efficacy of L2 performance. To that end, we developed a rating scale for measuring functional adequacy
(FA). In order to investigate the validity, reliability, and applicability of the rating scale, a number of studies are reviewed
in which FA was assessed by both expert and non-expert raters, in different learning contexts, for L2 and L1, involving various
source and target languages, proficiency levels, task types and modalities. We discuss perspectives and challenges for the use of
the FA rating scale, particularly with regard to task-based language assessment (TBLA).