Diet quality indices (DQIs) are widely used tools in nutritional epidemiology, yet some key aspects are often overlooked in research applications. The most frequent metrics in the literature are based on dietary guidelines or empirically derived dietary patterns, using nutrient and/or food-based approaches, and empirically derived dietary patterns. As dynamic tools, DQIs evolve alongside advances in health evidence and offer flexibility for adaptation to various contexts and populations through robust validation processes. They provide an effective means to investigate associations between diet and health outcomes and can be used as indicators to track vulnerable areas and strata in different populations. When applied thoughtfully and interpreted carefully, DQIs serve as powerful instruments for public health research and policies, offering nuanced insights into diet-related health risks and opportunities and supporting decisions to best allocate resources for food and nutritional security. This chapter underscores both the potential and limitations of DQIs, highlighting their pivotal role in supporting evidence-based nutrition and public health strategies.