The difference between modernization theory and dependency theory is that one explains how developed states operate and grow, while the other explains the exact opposite. Modernization theory explains the progress and development of technology in more stable regions of the world. Dependency theory observes and explains the effects modernization in one region has on other parts of the world. Dependency theory is more inclusive than modernization theory.
Modernization theory says that a focus on education, technology and mass media is a major reason that developed regions pull ahead from others. However, it does not explain on a global scale where the resources, labor and power come from to makes this development possible. In modernization theory, all of the progress a region or state makes comes from a collective internal effort. Dependency theory picks up the pieces and connects the dots where modernization theory leaves room for interpretation.
Dependency theory states that it is the unleveled distribution of power in global politics and markets that keeps underdeveloped regions in poverty. Dependency theory looks closer at how international economics play into the underdevelopment of states and the nations within them. Modernization theory and dependency theory are really two sides of the same coin when explaining the history of international relations.