The three essays of this dissertation focus on estimating the heterogeneity in immigrants' performance in the host country's economy across skill levels. This dissertation research provides new estimates and perspectives on immigrants' impact on host countries' economies using cross-country and country-specific data. The first essay examines the causal linkage between ethnic networks and bilateral trade across different skill groups of immigrants and products' quality levels. The results suggest that a higher level of human capital provides high-skilled migrants with opportunities to trigger more high-quality trade than the low-skilled migrants. Low-skilled migrants also use their home country knowledge to their advantage to trigger more low-quality trade compared to their counterparts. In addition, I find evidence of the positive secondary ethnic network effect of third-country nature on trade. This effect is stronger for low-quality products and more so through low-skilled migrants. The second essay investigates the impact of immigrant networks on the extensive and intensive margins of US exports. This essay contributes to the literature by exploring variation across different skill groups of immigrants and products' quality. The results indicate that immigrants affect both intensive and extensive margins of trade. However, in the case of intensive margin, the effect is higher for high-quality products, while in the case of extensive margin, the effect is higher for low-quality products. High-skilled immigrants trigger more high-quality trade in both aggregate export and intensive margin compared to their counterparts and vice versa. Both skill groups show a stronger network effect for low-quality products on the extensive margin. The third essay explores the heterogeneity in the assimilation process of new immigrants in the US regarding their occupational mobility. While this study's research question is quite different from that of my first two papers, it similarly focuses on variation in immigrants' performance. I find differential outcomes of occupational mobility and the speed of assimilation for different groups of immigrants. The analysis indicates that immigrants working in an occupation with licensing requirements experience less downgrading than those working in unregulated jobs after obtaining permanent residency. The study also finds that living within the home country community can hinder the assimilation process in the long run.