Chapter 6 analyses the ecological mechanisms, and implications, of intraspecific trait variability (ITV) and some key approaches to take ITV properly into account in modern trait-based analyses. The different sources of ITV, genetic variation, epigenetic effects and phenotypic plasticity, are discussed and put in the context of species evolution, adaptation to environmental conditions, species distribution potential (including invasive species) and the effects of species on multiple ecosystem properties and trophic interactions. Different tools are provided to quantify how strong ITV affect ecological patterns. A comparison of within- vs between-species trait variability in a community is discussed. Tools showing how strong the effect of changes in species composition (turnover) compared to ITV along environmental gradients are provided. Finally, methods considering ITV to quantify trait differences between species, via trait overlap in trait probability distributions, are discussed in the light of modern tools measuring functional diversity across different scales