This paper examines the effect of changes in migration determinants on the skill level of undocumented immigrants from Mexico. We focus on the effect of changes in economic conditions, migrant networks, and border enforcement on the educational attainment of Mexican-born men who cross the border illegally. Although previous research indicates that illegal aliens from Mexico tend to be unskilled relative to US natives and that economic conditions, networks and border enforcement affect the size of illegal immigrant flows across the border, the interaction of these variables has not been investigated. Results from hazard models using data from the Mexical Migration Project indicate that improvements in US and Mexican economic conditions are associated with relatively less-skilled undocumented immigrants. Stricter border enforcement is associated with higher skill levels. Access to a network of previous immigrants appears to lower the cost of migrating but has no differential effect by skill level.