As educators move to introduce computing in K-12 classrooms, the issue of assessing student learning of computational concepts, especially in the context of introductory programming, remains a challenge. Assessments are central if the goal is to help students develop deeper, transferable computational thinking (CT) skills that prepare them for success in future computing experiences. This chapter argues for the need for multiple measures or 'systems of assessments” that are complementary, attend to cognitive and noncognitive aspects of learning CT, and contribute to a comprehensive picture of student learning. It describes the multiple forms of assessments designed and empirically studied in Foundations for Advancing Computational Thinking, a middle school introductory computing curriculum. These include directed and open-ended programming assignments in Scratch, multiple-choice formative assessments, artifact-based interviews, and summative assessments to measure student learning of algorithmic constructs. The design of unique 'preparation for future learning” assessments to measure transfer of CT from block-based to text-based code snippets is also described.