Numerous quantitative genetic studies of yield of maize, conducted at North Carolina State University, are reviewed in order to provide a joint interpretation of the various kinds of evidence found. Estimates of variance components and the comparison of observed and expected response to selection have led to the following conclusions: 1. Sufficient additive genetic variance for yield exists within many locally adapted maize populations to permit improvement by selection. 2. Average dominance for genes affecting yield of ear corn is in the range of partial to complete. 3. Overdominance is not a prevalent source of genetic variation for yield. 4. Linkage disequilibrium can result in effects which mimic overdominance. These effects diminish with random mating. 5. Epistatic variance (i. e., genetic variance not accounted for by additive and dominance variances) is negligible in the varieties, Jarvis and Indian Chief, and in the F1 hybrid. 6. Short term predictions of response to full-sib and to reciprocal selection, based on genetic theory and computed from reasonably precise estimates of variances, are reasonably reliable.