Chapter 5 provides concepts and tools to characterize the functional trait structure of communities. Various indices are introduced, mainly community weighted mean (CWM) and various functional diversity (FD) indices. The power and the limitations of CWM are discussed. Various indices of FD, which expresses the extent of trait differences between organisms, are introduced to simplify their use and interpretation. The broad classification into ‘families’ of indices, i.e. functional richness, functional evenness and functional divergence, is discussed. A selection of indices, with their ability to provide a measure of FD at different scales (alpha, beta and gamma diversity), is discussed, together with other emerging components such as functional redundancy and functional rarity. A discussion on existing R tools, with their potential tricks and problems, is provided, also with examples available in the R material accompanying this chapter.