In chemistry, congeners are chemical substances "related to each other by origin, structure, or function".

Any significant quantity of a polyhalogenated compound is by default a blend of multiple molecule types because each molecule forms independently, and chlorine and bromine do not strongly select which site(s) they bond to.

Similarly polychlorinated dibenzodioxins, polychlorinated dibenzofurans, polychlorinated terphenyls, polychlorinated naphthalene, polychloro phenoxy phenol, and polybrominated diphenyl ethers (PBDEs) (pentabromodiphenyl ether, octabromodiphenyl ether, decabromodiphenyl ether), etc. are also groups of congeners.

Congeners can refer to similar compounds that substitute other elements with similar valences, yielding molecules having similar structures. Examples:

Structural analogs are often isoelectronic.