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Species coexisting in ecological communities interact in multiple ways to form complex networks. We review the growing literature on ecological interaction networks to address several key issues about this conceptual and methodological approach. We start by asking the most basic question: Why study networks and whether a network approach is (or is not) useful to understand the ecology of interacting species, the functioning and stability of the communities they belong to, and their response to global change drivers. We also discuss the multiple meanings of network nodes as individuals, populations and species, the different ways of quantifying node roles, the multiple meanings of links as presence/absence of interactions, per capita interaction strengths and species-level effects, and the available approaches to study networks with different types of interactions. Then, we review the structural patterns emerging in ecological interaction networks and the mechanisms driving network structure and function, identifying both what we already know and the knowledge gaps that we still need to fill in. We also discuss sampling effects and their influence in distorting observed network patterns. Finally, we review how different drivers of global environmental change influence the structure, dynamics and stability of ecological networks. With this review we hope to offer a balanced overview of what we have learned in the study of ecological interaction networks and point to several key avenues of research for the next decade.