Macrophages are sentinels and regulators of the immune system. They play pivotal roles in attacking foreign insult and maintaining organismal homeostasis. Upon tissue damage or infection, they can engulf apoptotic cells and pathogens, produce immune effector molecules and regulate the immune dynamics for wound healing. At steady state, they are strategically located throughout the body for tissue surveillance. The absence or dysfunction of certain sub-types of macrophages may lead to tissue injury, auto-immune diseases, neurodegeneration1, metabolic syndrome, or tumor metastasis. Therefore, understanding the mechanism how macrophages are recruited, infiltrate, and how they function in different contexts may provide useful insight to develop diagnosis methods and treatment strategies. In this talk, we will share some of our recent results on the imaging of macrophage in vivo. We track their activity in bleeding environment and see the change of their polarity in vivo.