This book examines advantages and disadvantages of national and international investments regarding the convergence of the capital markets and their correlation. The analysis focuses on diversification benefits of investig across countries versus investing across industry sectors. It provides an interpretation of the Markowitz's Mean-Variance analysis. It shows the contribution of the exchange rate risk to the total risk of stock indices portfolio using monthly returns for the last 15 years. It undertakes a correlation analysis among different country and industry indices. The examination of different market situations such as bear and bull markets confirms instability of the correlation coefficients and exhibits their undesirable properties under these special market conditions. It provides an analytical and geometrical interpretation of the traditional spanning tests (likelihood ratio, Wald and Langrange multiplier) as well as the alternative for the traditional tests: step down procedure. The empirical results suggest that adding of the country indices to the portfolio of the investor leads to better portfolio performance than the adding of the industry indices.