Immigration is a leading policy concern for many countries around the world. Improvements in technology, the fluidity of capital, increases in foreign direct investments, and other economic developments have increased the economic interdependence among countries. Lower-cost transportation and faster communication have facilitated globalization of the world's economies, as well as increased exposure to foreign languages, culture, and social networks. Furthermore, globalization has brought about greater awareness of economic opportunities that exist in other parts of the world. As such, more affluent countries face high levels of demand for participation in their economies by potential international migrants. And while political opinions regarding immigrants are frequently polarized, the transnational flow of migrants also creates numerous policy challenges, as well as opportunities, for receiving countries. For Members of Congress, immigration policy has been one of the main legislative issues for several years. With a near-consensus view that the current system is broken, legislators have introduced numerous pieces of legislation—from incremental to comprehensive—intended to provide solutions. Crafting policies to establish a politically acceptable migration level, in addition to attracting the most desirable migrants, remains a coveted goal in migration management. Yet, determining an optimal immigration policy has grown increasingly complex as economic, cultural, and security pressures all compete for political consideration. Thus, as Congress works to develop legislation that would provide incentives and deterrents for and against migration, it becomes important to have a foundation for understanding migratory behavior, as well as the major policy challenges linked to international migration. This report serves as a broad overview of the standard theory of international migration and offers a brief synopsis of the major immigration-related policy challenges relative to the legislative debates in Congress. The overview provides several possible issues for Congress as it considers new legislation on immigration reform, including (1) how new immigration legislation might affect migratory behavior and (2) the possible effects of increased or decreased migration on related policy issues. Distributive questions, including the domestic cost of international migration, lie outside the scope of the present analysis. For most advanced industrialized countries (AICs)—a term that generally refers to the member states of the Organisation for Economic Co-operation and Development (OECD) —the admission of international migrants constitutes something of a dilemma. On one hand, the admission of migrants is a desirable course of action because it mitigates labor shortages, imports human capital, and increases revenues through a broader income tax base and higher levels of consumption. On the other hand, from a political perspective, the admission of migrants may potentially result in a host of undesirable byproducts, such as the displacement from jobs of home country nationals, potential increases in crime and crime networks, cultural tensions, the increased risk of terrorist exposure, and, for some domestic groups, a lower standard of living. In most countries, a balance is struck between open or closed borders. Competing interests lobby for changing immigration laws and institutions in order to better suit their economic, cultural, and political preferences. In the United States, for example, the groups advocating restricted immigration flows frequently lobby legislators in an attempt to tighten border security and prevent the inflow of illegal immigrants, which these groups perceive as important to protect both the U.S. economy and security. Some groups, however, view these inflows as cultural threats. Higher education institutions, in contrast, tend to lobby for the admission of highly skilled foreign nationals who may serve as faculty members or as students—thereby embracing both immigrant labor and dynamic multiculturalism. Moreover, the business community frequently advocates for the admission of certain types of immigrants who may provide needed skill sets or lower-cost labor. Policy makers must weigh these considerations and many others in determining who should be admitted and who should be restricted. In OECD countries, a common strain that runs among the countries is an emphasis on three different areas: family connection, economic desirability, and humanitarian support. Numerous countries will allow for the reunification of immediate family, thus making family connections a common form of legal permanent entry. As shown in Table 1 , the family-based migration category represented more than half of all migrants in at least four OECD countries in 2006. When combined with the family admission of employment-based workers, family migration accounted for the majority of migrants in at least seven of the OECD countries in Table 1 —a total that does not include potentially family-related migration under the free movement regime of the European Economic Area. In terms of humanitarian admissions, these migrants tend to include asylees, refugees, and other migrants with protected status. In general, these groups have experienced declining admissions to OECD countries in recent years. Table 1 shows that this category tends to represent the smallest of the major admission groups. A group that has been the focus of many policymaking efforts in the United States in recent decades has been economically based migrants. While there may be demand for any number of occupations in an advanced economy, policy concerns often dictate that there be as little displacement or wage depression of native workers as possible. Although economists often decry such concerns as producing inefficiencies, it is frequently regarded by political scientists as politically prudent. In large part due to political forces, most countries have a relatively permissive policy toward highly skilled individuals, while the policy toward less-skilled individuals tends to be more restrictive. Furthermore, there are usually additional requirements placed on an applicant, such as an employment offer from a domestic employer. These requirements are generally placed on individuals to ensure that foreign nationals contribute to the domestic government's revenues rather than costs, and in this regard, higher-skilled individuals are perceived as being lower-risk admissions. With the exception of certain trafficking circumstances, all migration is based on a decision-making process by the individual migrant (or in some cases by the migrant's household). Even when migration is forced—as is the case with refugees—or takes place as a mass activity, some set of choices will generally occur. "If," "when," "where," and "how" questions of migration are commonly referred to as "push/pull" factors. Essentially, while some factors appeal to potential migrants to "pull" them toward another country, other circumstances tend to "push" them away from their place of residence. These factors are generally distinguishable in that the push factors are normally domestic circumstances (e.g., home country unemployment, poverty, limited opportunity, cultural oppression), while the pull factors are circumstances occurring outside the domestic arena (e.g., employment opportunities, family resettlement abroad, educational pursuits, social opportunities). A "push/pull" framework is rooted in a basic economic model of international migration. Under this model, at a very basic level, each individual is seen as performing a cost/benefit calculation based on the information he or she has available on the various push/pull factors to determine whether migration will provide a net gain or loss. Assuming that migration will provide a net gain (or a sufficient net gain), that individual may choose to migrate. The basic premise of economic models of migration can be expanded to include the concept of deriving utility from such diverse actions as family reunification and cultural change, for example. Neoclassical economics generally predicts that labor flows from low-wage/labor-rich states to higher-wage/labor-poor states. Broadly, individuals engage in economic migration (including educational pursuits), social migration (such as family reunification or alternative cultural pursuits), or political migration (such as asylees and refugees). For each of these categories, the more important push/pull elements to decision making vary, although the categories are not mutually exclusive and multi-motivational behavior and migration type overlap may occur. Moreover, some push/pull factors will serve as broad predictors for migration, while others will likely matter only for individuals engaging in a certain type of migration. Despite there being several possible motivations to migrate, the range of choices available for an individual to migrate are limited. Limitations occur in part because of the constraints placed on human behavior by institutions. The link between the behavior of individuals and the rules that govern them make up what economists and political scientists refer to as institutionalism . Institutions, as defined by economist and Nobel Laureate Douglass C. North, "are the humanly devised constraints that shape human interaction." In other words, institutions are the so-called "rules of the game." Institutions are not merely the formal rules (meaning those rules that are written down, such as laws and regulations), but also the informal rules (such as norms and values). While most observers recognize that formal institutions may modify migratory behavior, informal institutions may do so as well. Consequently, it is important for policy makers to recognize the effects of informal institutions, such as cultural expectations and traditions, on different migrant groups. Institutions, both formal and informal, are important not only because they constrain behavior, but also because they may facilitate behavior. Policy makers have long employed this principle in developing taxation policies whereby individuals or corporations receive tax reductions or write-offs in exchange for certain behaviors that are deemed socially beneficial (e.g., reduced pollution, community activities). For example, incentives may be developed that would reduce the tax burden for workers willing to migrate to remote geographic regions that contain valuable natural resources. Typically, these manipulations will affect formal institutions (because norms and values are much more difficult to alter). The resulting questions for policy makers in terms of developing migration policy are therefore as follows: Whose migration do they wish to facilitate? Whose migration do they wish to constrain? By what method do they wish to go about facilitating and/or constraining the movement of people? The first two questions are largely a matter of economic needs and cultural values, and are therefore determined through political forces in each country. Once the targeted migrants are identified by a government, the third question comes into play. The government approach to migration policy normally develops into a dual-pronged set of institutions built on constraining policies and facilitating policies.