Chapter 4 focuses on different approaches to studying the relationship between environmental conditions and trait variability, both within and between species. First, an historical perspective on species distribution and adaptations along environmental gradients is provided. The concept of environmental gradients is then discussed in depth, with distinctions between different types of gradients. This leads to a description of the widely applied trait-filtering metaphor, describing how environmental conditions filter out species with traits less adapted to a given habitat. The distinction between different types of analyses relating traits to environmental conditions is discussed (species- vs community-level analyses). Examples of these analyses are provided in the accompanying R material for this chapter. The importance of species-level analyses is highlighted, particularly in terms of species’ trait-fitness relationships and the parameterization of species distribution models.