Analysis of plant diversity aims to understand the organisation and the variability of biological populations within ecosystems. In classical analysis, individual plants are firstly identified on the basis of morphological/cytological parameters, then biodiversity is evaluated tacking into account presences/absences, abundances and densities of plants. Although morphological parameters are easily accessible, they provide limited precision on the differentiation between individuals that share a high similarity. This book provides illustrations on different chemotaxonomical criteria helping to understand complex structures of plant diversity. It focuses particularly on the chemotaxonomic usefulness of phenolic compounds (phenylpropanoids, flavonoids, etc.) and sesquiterpenes in analysis of plant chemical polymorphisms at different systematic levels (from family to variety and chemotype via genus and species).