Development economics is a key branch of economics which attempts to highlight aspects in the development process of developing countries. It is defined as the differing theories concerning growth, poverty, institutions, and inequality in less developed countries. In order to answer the central question of the paper: “To what extent do the theories of development economics help to understand the economic reality of developing countries”, this paper explores and assesses the different theories of development economics, and applies them to the economies of Nigeria and China, highlighting the various aspects of the theories that apply to the actual conditions of the respective countries. Nigeria is among the most developed countries in Africa, which has experienced considerable economic growth since the early 20th century and is now considered as Africas biggest economy. However, in spite of being a nation blessed with large oil reserves, the economy has performed relatively poorly in the past three decades. In contrast to Nigeria, China has experienced rapid economic development over the last three decades, to become one of the largest economies in the world. By applying the theories of development to the growth of the Nigerian and Chinese economies, the evidence suggests that the extent to which the development theories help to understand the economic realities of developing countries depends on the country in question, as the theories cannot be applied to every developing country equally, because each country has different political and cultural dynamics, creating various environments that influence the direction of the economic development process.