This chapter describes food frequency questionnaires (FFQs), which serve as the most appropriate and widely utilized tools for nutritional assessment in epidemiological studies. It elucidates the rationale behind their suitability and extensive application, while providing a historical overview of FFQs, their significance in nutritional epidemiology, and their structural and design elements. The chapter discusses existing methods for selecting food items for the food list, as well as strategies for categorizing and grouping these items. It also examines the various types of FFQs, their administration methods, and their advantages and limitations in comparison to other assessment methods. Recognizing the diverse factors that influence the diets of different populations underscores the necessity of developing or adapting FFQs to meet specific needs. The chapter further addresses the critical aspects of validation and reproducibility of FFQs, detailing the validation processes and available methodologies. Finally, this chapter emphasizes the impact of mental health on dietary habits, exploring the bidirectional relationship between diet and mental disorders, along with the potential mechanisms through which diet may contribute to the pathogenesis of these disorders. It highlights the significance of various factors shaping the eating habits and dietary patterns of affected individuals, the nutritional challenges they encounter, and the essential need for valid and reliable FFQs tailored to this demographic. Such adaptations are vital for accurately evaluating their nutritional status and advancing research in the emerging field of nutritional psychiatry.