Adhesion is the surface attraction between the surfaces of two materials. The term is sometimes used to describe the tendency of two adjacent surfaces, which may be of different chemical compositions, to cling to each other.  Natural adhesives have been replaced in many uses by synthetics; but animal glues, starches, gums, cellulose, bitumens, and natural rubber cements continue to be used in large volumes. The use of bioadhesives in medicine is discussed in the section on Health Products. There are several advantageous properties for a biological glue. These  include:

	Good adhesive and cohesive strength in moist environments
	Elasticity and flexibility
	Compatibility with tissues, cells and organic molecules
	Porosity: micro-porosity to permit  exchange of gas and  nutrients

The market for bio-adhesives is significant, with applications seen in medicine, textiles, manufacturing, construction industries, etc.. Adhesive biomaterials come from an exceptionally wide range of sources. 

Sources of Bioadhesives:

Animal-derived (terrestrial): materials produced by humans using flora and fauna as raw materials:

	Mammals: adhesives made from collagen, (Skin/bone) with sugar / glycerol added for flexibility.
	Fish: similar to  above but with improved temperature resistance, resistance to water compared to above
	Egg whites: used for some veterinary casts for fractures
	Casein glues: protein-based, made from milk precipitated with acid.

In-vivo Adhesives:

Rubber tree  Natural rubber is known for its excellent flexibility and resiliency.  These adhesives  are naturally-occurring polyisoprene elastomers recovered from the sap of rubber trees (hevea brasiliensis) and certain other trees and plants. Water-based adhesives derived from natural rubber latex, available commercially at approximately 60% solids, are widely used for carpet backing and carpet installation, self-sealing envelopes, and flooring and tile bonding. The rubber latex is commonly formulated with thickeners, such as cellulose or polyvinyl alcohol, tackifiers, such as aqueous dispersions of hydrocarbon tackifiers or high boiling hydrocarbon solvents, inorganic fillers, and alkali viscosity stabilizers. Additionally, fungicides are frequently included to prevent bacterial attack.)

Triodia sp.(Australia) Several adhesives were used. In central Australia, resins beaten from the foliage and stems of Spinifex grasses () were gathered together with a hot stone. The resultant mass was used to secure spearheads etc.. This resin is thermosetting - softer when warm - hard when cooled. 

The leaves of the Sundew (Drosera capensis) are covered with dense, gland-tipped hairs coated with a glue-like substance.  The plant uses this to capture midge flies, later digested by .

Algae:  Studies are underway examining the application of algal bioadhesives for wound closure and drug delivery. Targets for this research incoude novel, surgical tissue adhesive compounds, and physico-chemical, biochemical, molecular and cellular methodologies defining  the algal bioadhesion process.

Spider webs: People have been studying spider silk for hundreds, (perhaps thousands) of years, attempting to understand how the nanometer-sized biodegradable threads can be stronger by weight than high-tensile steel and elastic enough to stretch up to 10 times their initial length. These superior fibers could potentially be used to create lightweight bullet-proof body armor, tough biodegradable bandages, and even artificial tendons

Monarch Butterfly: Monarchs produce a fast drying, water-insoluble, super glue used to affix eggs to the leaf surface. The glue is produced by a pair of long tubular tissues known
as collaterial glands, which empty into the common oviduct.

Frogs: Australian frogs of the genus Notaden secrete a sticky adhesive when provoked, as an anti-predator defense.

Mussels (Atlantic ribbed mussel Geukensia) collagen/silk mix with a blended, rather than abrupt interface between the two materials. Has both significant toughness combined with flexibility. The foot of the mussel, for example, releases an exudate that is basically a complex phenolic compound that interacts with the biofilm on the underwater surface to form an adhesive. Recent research at Purdue University indicates this adhesive requires iron, a significant development in natural adhesive formulations.

Bacterial Adhesives: 

1. There is wide range of bacterial adhesives, allowing these microorganisms to adhere to a variety of surfaces. Microbial adhesion / contamination can cause serious problems in the medical, dental and food science fields.  Bacterial adhesion begins with the long-range, reversible interactions between a bacterium and a substrate. Once a bacterium is in close proximity to a surface, it can establish short range, permanent interactions.  Bacteria cement themselves to the surface, forming a slimy layer known as a biofilm.
2. Bacterial exopolysaccharides (carbohydrate-based materials) are the main component of the biofilm glycocalyx, which has also been described as a "slime layer".  In most species, the glycocalyx is predominantly anionic and creates an efficient scavenging system for trapping and concentrating essential minerals and nutrients from the surrounding environment

Clearly there are innumerable sources for bioadhesives; adhesives that may be biocompatable with surgical procedures or simply used to glue household items. Bioadhesion continues to be a highly active area of bioresearch. Additional information is found the Health section.