Endo-exo isomerism is a special type of isomerism found in organic compounds with a substituent on a bridged ring system. The prefix endo is reserved for the isomer with the substituent located closest, or "syn," to the longest bridge. The prefix exo is reserved for the isomer with the substituent located furthest, or "anti," to the longest bridge. Here "longest" and "shortest" refer to the number of atoms that comprise the bridge. This type of molecular geometry is found in norbornyl systems such as dicyclopentadiene.

Endo exo isomerism in norbornyl systems

The terms endo and exo are used in a similar sense in discussions of the stereoselectivity in Diels-Alder reactions.

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