Because of their importance as pollinators of wild and cultivated plants, little concern has been paid to alien bees when entering a new ecosystem. Hence, approximately 80 alien bee species worldwide have spread outside their native ranges. Here, we explored the main impacts of alien bees on native bees through competition for food or nesting resources, interference, pathogen spillover, and genetic contamination. Implications for native bee conservation are also discussed. In addition, in this chapter, we developed a particular focus on the first alien bee that colonised Europe, Megachile sculpturalis. We emphasised the main knowledge gaps and important trends for future research. Finally, avenues for managing alien bee species and preventing their introduction are provided.