Book excerpt: Successful speaking and understanding hinges on the almost effortless capacity of speakers to decode and build dependencies among words in a sentence, based on covariance in some specific feature(s). Whenever two features covary, an agreement relation is established. Agreement is a widespread and varied phenomenon: its pervasiveness in some languages contrasts with its near absence in others, which poses a challenge for linguists and psycholinguists that attempt to explain the mechanics of its representation, processing and acquisition. Agreement has been extensively investigated from a theoretical perspective, but also from the point of view of psycholinguistics and the cognitive neuroscience of language. Theoretical linguistics has provided an articulated system of structural representations and computations on which the establishment of agreement relations hinges, while psycholinguistics and cognitive neuroscience have aimed at unveiling the algorithms that underlie the use of these computations and their behavioral and neuro-physiological bases. The goal of this Research Topic is to draw together multiple and interdisciplinary work to highlight the state of the art in the study of agreement and propose new perspectives on this research topic. Publisher’s note: In this 2nd edition, the following article was added: Mancini S, Caffarra S and Nevins A (2021) Editorial: Featural Relations in the Brain: Theoretical and Experimental Perspectives on Grammatical Agreement. Front. Psychol. 12:754430. doi: 10.3389/fpsyg.2021.754430