The Generalized System of Preferences (GSP) is a unilateral trade liberalization program in which developed countries offer non-reciprocal tariff reductions (tariff preferences) on certain products imported from designated developing and least developed countries. GSP is considered an important tool in the World Trade Organization's approach to development. This dissertation--composed of three essays--explores whether low-income countries have achieved an increased access to high-income markets as a result of these non-reciprocal tariff preferences offered to their exports. The first essay provides an overview of the GSP program. The second essay presents an evaluation of the GSP program by considering the products and markets where low-income countries' exports are concentrated. Using a theoretically consistent gravity equation for primary and processed agri-food trade over the period 1962-2010, the results illustrate that the GSP program and modifications of it have delivered significant positive effects in developing countries' exports to developed country markets in agricultural trade but not necessarily so in non-agricultural goods. The third essay develops two theoretically founded novel indices to measure preference margins offered by high-income countries to low-income countries through tariff reduction. One index captures the restrictions bilateral tariff rates impose on market access conditions of a country as compared to the most favored nation rate, called the Exponential Trade Restrictiveness Index (ETRI). The other index captures the relative ease with which a country can access foreign markets compared to its competing suppliers, called the Exponential Relative Preferential Margin (ERPM). Then, these two bilateral indices are used to develop a model of sector-based bilateral trade to re-evaluate the Generalized System of Preferences (GSP) in terms of relative market access preferences. The results show that the GSP has increased relative market accessibility for low-income countries and in turn boosted exports from these countries by 26 to 28 percent. / Ph. D.