Topology is the study of the properties of objects and networks that are preserved under deformations that do not break connections/surfaces or require surfaces/edges to pass through one another. Chemical topology applies these same ideas to molecules. At the simplest level, constitutional isomers are topologically distinct as they differ in the network of atoms. More interesting topological isomerism arises when structures contain the same atomic connections, 2 the most famous examples of which are Möbius ladders (topological isomers of the untwisted macrocycle), molecular knots (topological isomers of the unknotted ring), and [2]catenanes (topological isomers of two non-interlocked rings). These structures have non-planar graphs in that there is no two-dimensional projection of their structures in which bonds do not cross over one another at least once and this property is topologically invariant in 3 dimensional spaceno matter how the structure is distorted, even drastically altering the geometry around atoms, it is impossible to achieve a planar graph without breaking bonds. Topologically non-trivial structures can in turn display molecular chirality in the absence of covalent stereogenic units. Depending on their topology, Möbius ladders 7 and molecular knots 8 are chiral regardless of their constitution as this relies only on the pattern of bond crossing points. Although simple [2]catenanes do not display unconditional topological stereochemistry, as first recognized by Wassermanan and Frisch, 10 they display stereochemistry conditional on the constitutional symmetry of the rings; rings that are "oriented" as a result of the sequence of atoms in the cycle (Cnh symmetry) give rise to topologically chiral catenanes (Figure ). The absolute stereochemistry of topologically chiral Mobius ladders, knots and catenanes is invariant under topologically allowed deformations in 3 dimensional space. We recently identified 11c several "missing" stereogenic units that arise in interlocked molecules and give rise to classes of chiral rotaxanes and catenanes that had yet to be discussed or synthesized. An example that presents particular linguistic problems are [2]catenanes in which one ring is oriented (Cnh) and the other is bilaterally symmetric (C2v) (Figure ). If coconformational motion is unhindered, their time averaged structure is achiral but any co-conformation in which the oriented ring does not lie on the internal mirror plane of the C2v ring is chiral.