Frontal sinusitis is inflammation or infection of the sinuses located just behind the eyes and in the forehead. The sinuses are a system of connected hollow cavities in the face that contain air and a thin layer of mucus. All sinuses produce mucus that moisturizes the airways and drains into the nasal passages. If the frontal sinuses are inflamed or infected, they cannot drain mucus efficiently, and this can make breathing difficult. It can also lead to a feeling of increased pressure around the eyes and forehead.