One nation does not act independently when it comes to their economy. Nations depend on each other for resources, support, and more. There are different models and theories that try to explain the economic relationships that nations have with one another. For example, Rostow’s Modernization Model, Dependency Theory, and Wallerstein’s World Systems Theory Model attempt to explain the economic path of nations in comparison to each other. The different sectors of the economy are primary, secondary, and tertiary. They categorize what types of jobs the population is engaged with. In relation to economic interdependence, agglomerations, or a concentration of related economic activities, are also prevalent as nations hone in on a particular activity.