There is a growing interest in conceptualizing, defining, and assessing what are often called 21st century skills or deeper learning competencies. The purpose of this literature review is to explore the conceptualizations, definitions, and understandings in the research literature related to critical thinking. Key initial questions include: What is critical thinking? How is critical thinking related to other success skill concepts? And to what extent does critical thinking develop over time? This foundational information will then be used to examine: (1) instructional approaches to promote critical thinking; (2) benefits of critical thinking on valued student outcomes such as student learning; and (3) ways teachers can collect evidence that reveals the benefits of student critical thinking outcomes using student artifacts and other appropriate measures.