Fog is a visible aerosol made up of small water droplets or ice crystals floating in the air above or near the Earth’s surface. When water vapor condenses into microscopic water droplets floating in the atmosphere, fog starts to form. Depending on its type and content, fog dynamics can have a variety of consequences on the climate and air quality. Fog behaves similarly to a thermal blanket. It reduces temperature swings and creates warmer conditions in foggy regions by retaining heat during the night and keeping it from leaking into the atmosphere. When fog lifts and evaporates, it can add moisture to the atmosphere, influencing local humidity levels and contributing to the hydration of the surrounding environment. Fog can create localized climate conditions, known as microclimates, which can significantly affect plant and animal life in the region. Fog can trap pollutants like particulate matter and gases close to the ground, leading to higher concentrations of these substances, especially in urban areas with heavy traffic and industrial activities. Dense fog significantly reduces visibility, creating hazardous conditions for transportation. This can lead to increased vehicle emissions as engines run longer in slow-moving or idling traffic. The microenvironment within fog droplets can facilitate chemical reactions, sometimes transforming pollutants into more harmful forms. This can alter the composition and toxicity of air pollution. Moreover, this chapter will highlight the formation of fog, the factors that affect fog formation, and the influence of air quality and climate on fog dynamics.