Understanding how molecular systems react, i.e. which reactions are possible under what conditions, is an essential part of chemical research and computational methods for exploring reaction space in an automated manner are continually being proposed. These methods can be divided into three different categories: (semi-)exhaustive searches, reaction template methods, , and meta-molecular dynamics (meta-MD) based approaches. Examples of (semi-)exhaustive searches include graph enumeration of products and enumeration of reaction coordinates. For these methods the size of the search space, and hence the computational cost, grows quickly with the size of the molecules. The reaction template approaches lie at the other extreme in terms of computational efficiency, and work by investigating only pre-determined reaction types. Though efficient and used extensively in atmospheric and combustion chemistry, this approach can be hard to generalise to other areas such as synthetic organic chemistry, though a recent attempt is encouraging. The meta-MD approaches explore reactivity via biasing potentials that force reactions and exploration of conformational space. The meta-MD approach can also be combined with the reaction coordinate approach as shown by Lavigne et al. In the non-exhaustive approaches the key question is whether they identify all relevant reactions for the problem at hand. At room temperature, this typically means all reactions with barriers less than ca 30 kcal/mol.