Oxygenic photosynthesis, arguably the most important biological process on our planet, empowers phototrophic organisms to convert sunlight into the chemical energy that sustains ecosystems. Yet, this very process faces a constant challenge: capturing solar energy efficiently while avoiding the detrimental effects of excess light. When light absorption outpaces the rate of carbon fixation, the inevitable consequence is a surge in reactive oxygen species (ROS), leading to photoinhibition and cellular damage. To combat this, plants and algae have evolved a sophisticated arsenal of photoprotective mechanisms, with non-photochemical quenching (NPQ) emerging as a central and highly regulated strategy. Chlorophyll a fluorescence has proven instrumental in deciphering these mechanisms, providing the foundation for powerful tools that unravel NPQ regulation and dynamics under changing environmental conditions. This chapter explores the fundamental principles underpinning NPQ and its pivotal role in enabling plants to thrive in dynamic light environments. Primary molecular mechanisms underlying NPQ and their key effectors are presented, alongside their physiological and ecological relevance. Existing controversies and knowledge gaps are also addressed, underscoring the need for continued research in this critical area. Furthermore, chlorophyll a fluorescence techniques, particularly pulse-amplitude-modulated (PAM) fluorometry, are highlighted as essential methods for investigating NPQ regulation and dynamics in response to environmental changes. Integrating these tools with molecular biology, microscopy, spectroscopy, and omics approaches provides a comprehensive understanding of how NPQ enables photosynthetic organisms to flourish in fluctuating light conditions. Understanding these mechanisms is essential for predicting plant responses to future climate conditions and for shaping sustainable agricultural practices that reinforce ecosystem resilience.