•Research into local adaptation at fine spatial scales has received little attention.•We develop new quantitative definitions for microgeographic adaptation and the spatial scale of adaptation.•Mechanisms promoting microgeographic adaptation include strong natural selection and reductions in gene flow.•Microgeographic adaptation can fundamentally alter understanding of ecological and evolutionary dynamics.•New frontiers are needed in spatial evolutionary research, particularly the lower spatial bounds of local adaptation. Local adaptation has been a major focus of evolutionary ecologists working across diverse systems for decades. However, little of this research has explored variation at microgeographic scales because it has often been assumed that high rates of gene flow will prevent adaptive divergence at fine spatial scales. Here, we establish a quantitative definition of microgeographic adaptation based on Wright's dispersal neighborhood that standardizes dispersal abilities, enabling this measure to be compared across species. We use this definition to evaluate growing evidence of evolutionary divergence at fine spatial scales. We identify the main mechanisms known to facilitate this adaptation and highlight illustrative examples of microgeographic evolution in nature. Collectively, this evidence requires that we revisit our understanding of the spatial scale of adaptation and consider how microgeographic adaptation and its driving mechanisms can fundamentally alter ecological and evolutionary dynamics in nature.