 The use of linear mixed effects models, LMMs, is becoming increasingly common in analyzing biological data, but their application requires knowledge of when to apply them and how to interpret results. This article provides a general overview of current methods for using LMMs in biology, highlights typical pitfalls, and offers practical solutions. It also discusses the use of information theory and multimodal inference in ecology. The article aims to serve as a code of best practice for applying LMMs to complex biological problems and model structures, improving the robustness of conclusions drawn from studies investigating ecological and evolutionary questions. This article was authored by Xavier A. Harrison, Linda Donaldson, Maria Eugenia Carriacano, and others.