Chapter 7 expands on the ideas already introduced in Chapters 4 and 6 on community assembly rules, understood as any constraint restricting the number and identity of the species observed in an assemblage. The different ecological processes behind such rules are discussed, together with the expected effects of these rules on trait patterns (trait convergence vs trait divergence) at different ecological scales. The importance of defining a proper reference species pool for assessing these mechanisms is explained. A further discussion is provided on the difficulty of ascertaining the specific ecological processes leading to observed patterns of trait variation without experimental approaches. This leads to introducing how null models and data randomizations can provide valuable insight into different assembly rules mechanisms, when proper care is given to considering the effect of scale and an adequate reference species pool. The R examples accompanying this chapter provide different tools to implement a variety of null models in combinations with functional diversity indices.