Epigenetics, the study of heritable changes in gene expression that occur without alterations to the DNA sequence, is pivotal in understanding fish biology. It offers valuable insights into developmental processes, environmental adaptability, and resilience. Fish, owing to their diverse habitats and exposure to various environmental stressors, serve as unique models for exploring epigenetic mechanisms. This chapter delves into the primary epigenetic processes in fish, including DNA methylation, histone modifications, and noncoding RNAs. DNA methylation, a key regulatory mechanism, influences gene expression, promotes genome stability, and governs cellular differentiation. Meanwhile, histone modifications and noncoding RNAs shape chromatin dynamics and regulate posttranscriptional gene expression, respectively. These processes play critical roles in vital functions like immunity and stress response. Epigenetic adaptations are fundamental to fish survival, enabling them to cope with environmental challenges. For instance, temperature fluctuations can trigger methylation changes that confer thermal resilience, while exposure to pollutants or toxins induces epigenetic alterations affecting growth, reproduction, and overall health. Additionally, the transgenerational inheritance of epigenetic marks enables fish populations to “remember” stressors, potentially enhancing resilience in future generations, although it may also result in maladaptive responses in rapidly changing environments. Recent advancements in age estimation using epigenetic clocks have significantly broadened the applications of epigenetics in fisheries management and conservation biology. By leveraging age-related DNA methylation patterns, these clocks provide precise, nonlethal methods for determining fish age, offering critical insights into population demographics. Accurate age prediction facilitates sustainable fishing practices, supports the management of overexploited stocks, and aids in the conservation of endangered species. Furthermore, epigenetic insights hold transformative potential in conservation biology and aquaculture, fostering resilience to climate change and habitat loss while promoting the development of desirable traits like disease resistance and stress tolerance. Ethical considerations and sustainability remain crucial in these endeavors.