Essay one is concerned with how and why an individual invests in human capital and how tax policy affects investment in human capital. We examine optimal investment in human capital and the effect of tax policy on human capital formation, and test several hypotheses derived from the theory using U.S. time-series data. Investment in human capital in terms of college enrollment rates is positively related to family income, rate of return to human capital, and unemployment rates, while it is negatively related to educational cost, and rate of return to physical capital. In addition, the average income tax rates show a negative effect on college enrollment rates. Essay two discusses human capital and economic growth. We first investigate the elasticities of substitution among inputs using the nested constant elasticity of substitution production function to focus on the so-called capital-skill complementarity hypothesis. We here compare two models: one is a model with human capital and raw labor, and the other is a model with higher skilled labor and lower skilled labor. In both models, the elasticities of substitution among inputs are very low, but the complementarity hypothesis is still weakly confirmed. Human capital turns out to be essential in achieving medium-term economic growth empirically. We also demonstrate the key role of human capital in the long-term steady state within the context of the endogenous growth model. Essay three considers the role of human capital on income distribution. Using the nested CES production function, we first derive factor shares, and then examine the relationship between functional and personal income distribution. An increase in share of labor income reduces overall income inequality, while an increase in share of transfer income has a negative effect on income distribution. Human capital, especially primary and secondary level of human capital stock, is a crucial factor in reducing income inequality. Finally, this study develops and presents new estimates of human capital stock in the United States, as well as annual earnings, and labor force by education level for the period 1947-1989. Data shows that the growth rate of GNP is very closely related to that of human capital stock. (Abstract shortened with permission of author.)