A bicyclic molecule is a molecule that features two fused rings. Bicyclic molecules occur widely in organic and inorganic compounds.

Fusion of the rings can occur in three ways:

·         Across a bond between two atoms - for example, decalin (also known as bicyclo[4.4.0]decane), has a C-C bond shared between two cyclohexane rings;

·         Across a sequence of atoms (bridgehead) - for example, norbornane (also known as bicyclo[2.2.1]heptane), can be viewed as a pair of cyclopentane rings that share three of the five carbon atoms; or

·         At a single atom (spirocyclic, forming a spiro compound)

Bicyclic molecules have a strict nomenclature. On its simplest level the parent hydrocarbon is the equivalent open carbon alkane. For bridged compounds, the prefix bicyclo is added, followed by, between brackets, separated by periods, and in descending order, the number of carbon atoms between each of the bridgeheads. For example in bicyclo[2.2.1]heptane the carbon frame contains a total of 7 atoms hence the parent name heptane. This molecule has three bridges having 2, 2 and 1 carbon atoms hence the prefix bicyclo[2.2.1]. For spiro compounds, terms like spiro[2.4] are used, indicating that there are 3- and 5-membered rings (because the spiro atom itself is not counted) meeting at the spiro atom.