Critical thinking is widely recognised as a central goal of contemporary education, yet it is often pursued through the analysis of external material while the learner's own thinking remains unexamined. This article argues that self-assessment and reflective writing are powerful and complementary tools for developing critical thinkers precisely because they turn the analytical gaze inward, prompting learners to evaluate their own reasoning, judgements, and learning. Drawing on theories of reflection, metacognition, and self-regulated learning, the article clarifies how reflective writing externalises and disciplines thought, and how self-assessment trains learners to apply criteria and evidence to their own work. It synthesises research indicating that self-assessment supports self-regulated learning and that reflective writing fosters metacognitive awareness, and it sets out practical strategies—learning journals, portfolios, guided reflective prompts, criteria and rubrics, and feedback that prompts further reflection—for integrating these practices into instruction. The article also considers the challenges of superficial compliance, assessment tensions, and the need for explicit scaffolding, concluding that, used deliberately, self-assessment and reflective writing cultivate the self-regulatory dispositions at the heart of critical thinking.