Immigration and gentrification are two sources of population change that occur in geographic communities. Immigration refers to the inflow of foreign-born residents, while gentrification involves middle- and upper-income residents moving into poor urban communities. Scholars have speculated that both types of population change may be related to crime rates. The nature of those relationships, however, is debated. Classic criminological perspectives, such as Social Disorganization Theory, suggest that these population changes are likely to result in increased crime rates. More recent perspectives proffer an opposing viewpoint: that immigration and gentrification may lower crime rates. Some research suggests that these opposing arguments may each draw empirical support but under differing social conditions or circumstances. Regarding the effects of immigration on crime, one theory is that immigration is most likely to be a crime-protective factor when it occurs in places where there is a well-established immigrant population base and institutional supports. And immigration may contribute to higher crime rates in places that lack the strong preexisting immigrant population and institutions. Study design variations also appear to impact the findings that researchers investigating the immigration-crime relationship have found, with longitudinal studies more consistently reporting the immigration works to reduce (rather than increase) crime. With respect to gentrification, scholars suggest that its effects on crime are likely to hinge on factors such as the racial composition of the place, the timing or stage at which the gentrification process is observed, or the degree to which gentrifying neighborhoods are surrounded by poor non-gentrifying neighborhoods or by other communities that have already progressed through the gentrification process.