It is the purpose of this research to evaluate northern red oak to: (1) Determine if any relationship exists between the quality of the sites on which the trees are grown (as measured by site index) and the proportion of various tissue types in the wood produced. (2) Quantify the proportions of tissue types in the species, across the geographic range and on a variety of sites. (3) Determine if relatioships exist between ring width and the proportion of tissue types. The following conclusions are drawn from this study. 1. There are quantitative relationships between site index and proportions of vessels, fibers, axial parenchyma, and rays in northern red oak; (a) vessel, axial parenchyma, and ray proportions are negatively correlated with site index, and (b) fiber proportions are positively correlated with site index. 2. A great deal of the variation in the four tissue types remains unexplained. The unexplained portion may be accountable by untested variables such as slope position and soil factors, or may be due to genetic factors. 3. The mass of data in objective 2 (tissue proportion variation within and between areas) should be valuable in establishing the variability factors for selection of anatomical attributes for tree improvement programs; e.g., the maximization of fiber content. 4. There are significant (P = 0.01) quantitative relationships between growth rate, as measured by ring width, and proportions of vessels, fibers, and axial parenchyma in northern red oak. Vessel and axial parenchyma proportions are negatively correlated with ring width, and negative curvilinear relationships are found between vessel proportions and ring width when average ring widths in excess of 4 millimeters are included in the regressions. Fiber proportions are positively correlated with ring width. The relationship between ray proportions and ring width for the species is negative but not significant. 5. Proportions of vessels plus axial parenchyma are generally the mirror image of fiber proportions, with ray proportions nearly constant. 6. The substantial portion of unexplained variation that persists after considering growth rate in regression analysis may possibly be accounted for by hormone levels. 7. The importance of tissue proportions on the mechanical properties of northern red oak warrants further study of the soil-site-physiology interactions on cell formation and differentiation in the species.