Instrument validation is essential for ensuring that research tools accurately measure their intended constructs, thereby enhancing the reliability and validity of study findings. This chapter presents the validation of two key instruments: the Assessment for Learning Strategy Questionnaire for Teachers (AfLSQ-T) and the Assessment for Learning Strategy Questionnaire for Students (AfLSQ-S). Construct validity was assessed through both Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA), supplemented by reliability analyses to evaluate internal consistency. Due to the larger student sample size, validation followed a sequential approach—first establishing psychometric properties for the AfLSQ-S, then for the AfLSQ-T. The validated questionnaires provide a well-tested foundation for investigating teachers’ and students’ perspectives on AfL strategies in subsequent phases of this research.