The Cognition Hypothesis postulates that more cognitively complex tasks can trigger more accurate and complex language production, thereby advancing second language (L2) development. However, few studies have directly examined the relationship between task manipulations and L2 development. To address this gap, this article reviews, via an analytic approach, nine empirical studies that investigated the impact of task complexity on L2 development in the domain of morphosyntax. The studies are categorized into two groups based on if they include learner-learner interaction or a focus on form (FonF) treatment provided by an expert interlocutor. The results indicate that the findings of the studies, albeit partially mixed, tend to support the predictions of the Cognition Hypothesis. More importantly, a further analysis reveals seven key methodological issues that need to be considered in future research: target linguistic domains, different types of FonF, the complexity of the target structure, task types, outcome measures, the use of introspective methods, and the need of more empirical studies and replicable study designs.