Propolis is a resinous mixture that honey bees collect from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the hive.
Propolis is used for small gaps (approximately 6 millimeters (0.24 in) or less), while larger spaces are usually filled with beeswax. Its color varies depending on its botanical source, the most common being dark brown. Propolis is sticky at and above room temperature (20 °C). At lower temperatures, it becomes hard and very brittle. For centuries, beekeepers assumed that bees sealed the beehive with propolis to protect the colony from the elements, such as rain and cold winter drafts. However, 20th century research has revealed that bees not only survive, but also thrive, with increased ventilation during the winter months throughout most temperate regions of the world.

Propolis is now believed to:
1. reinforce the structural stability of the hive
2. reduce vibration
3. make the hive more defensible by sealing alternate entrances
4. prevent diseases and parasites from entering the hive, and to inhibit bacterial growth
5. prevent putrefaction within the hive. Bees usually carry waste out of and away from the hive. However if a small lizard or mouse, for example, finds its way into the hive and dies there, bees may be unable to carry it out through the hive entrance. In that case, they would attempt instead to seal the carcass in propolis, essentially mummifying it and making it odorless and harmless.
6. Propolis is marketed by health food stores as a traditional medicine, and for its claimed beneficial effect on human health.
7. Natural medicine practitioners use propolis for the relief of various conditions, including inflammations, viral diseases, ulcers, superficial burns or scalds.
8. Propolis is also believed to promote heart health, strengthen the immune system and reduce the chances of cataracts. Old beekeepers recommend a piece of propolis kept in the mouth as a remedy for a sore throat. Propolis lozenges and tinctures can be bought in many countries. Though claims have been made for its use in treating allergies, propolis may cause severe allergic reactions if the user is sensitive to bees or bee products.
9. Some of these claims are being clinically investigated and several studies are published in the biomedical literature. Since the chemical composition of propolis varies depending on season, bee species and geographic location, caution must be applied in extrapolating results (as above).
10. As an antimicrobial
11. Depending upon its composition, propolis may show powerful local antibiotic and antifungal properties.
12. As an emollient
13. Studies also indicate that it may be effective in treating skin burns.
14. As an immunomodulator
15. Propolis also exhibits immunomodulatory effects.
16. As a dental antiplaque agent
17. Propolis is a subject of recent dentistry research, since there is some evidence that propolis may actively protect against dental caries and other forms of oral disease, due to its antimicrobial properties. Propolis can also be used to treat canker sores. Its use in canal debridement for endodontic procedures has been explored in Brazil.
18. As an antitumor growth agent
19. Propolis' use in inhibiting tumorigenesis has been studied in mice in Japan. In pre-clinical models of human breast cancer, propolis induces cell cycle arrest, apoptosis and reduces expression of growth and transcription factors, including NF-κB. Notably, caffeic acid phenethyl ester down-regulates mdr-1 gene, considered responsible for the resistance of cancer cells to chemotherapeutic agents