The determination of manganese (Mn) essentiality in plant growth by McHargue (1914–1922) focused the attention of plant nutritionists on this nutrient, and led the way for further groundbreaking studies. Since then, research into the concentrations of manganese that confer deficiency or toxicity, and the variation between- and within-plant species in their tolerance or susceptibility to these afflictions has proliferated. The symptoms of toxicity and deficiency have also received much attention owing to their variation among species and their similarity to other nutrient anomalies. The diversity of visual symptoms within a species that often confounds diagnosis has been attributed to soil conditions. Soil pH is one of the most influential factors affecting the absorption of manganese by changing mobility from bulk soil to root surface. In addition to research on manganese diagnostics, workers have also focused on the role of manganese in resistance to pests and disease, revealing economically important interactions that further highlight the importance of this nutrient in optimal plant production.