Sinuses are air spaces in the bones of the skull. They make the skull lighter and act as resonators for the voice. Like the nose, the sinuses are lined with mucous membrane (a slippery, protective lining). The mucus that the sinuses produce drains into the nose through narrow tubes. When the mucous membrane gets inflamed, it swells up and too much sticky mucus is produced, with the result that the sinuses and their drainage tubes get blocked. Pressure builds up in both the sinuses and the nose, leading to pain and congestion.

The most common reason for sinus pain and congestion is sinusitis - inflammation of the sinuses, which in turn is usually due to infection by viruses or bacteria, for instance in colds and flu. The infection spreads into the sinuses from the nose. The swelling and mucus-production is the body's way of attempting to flush out the invading "bugs."

Another common cause of sinus congestion and pain is allergies - or specifically, allergic rhinitis (also known as hayfever and house dust allergies). Allergic rhinitis happens when the immune system - which defends the body against viruses and bacteria - becomes hypersensitive and starts to respond to harmless substances such as pollen or dust, called allergens ("substances that provoke an allergic response"). The allergic reaction involves the release of inflammatory chemicals, like histamine, that cause blood vessels in the air passages to expand and extra mucus to be produced. Just like in a cold, this swelling and pressure creates congestion in the nose and/or sinuses, and sinus pain.