Osteoclasts are 'bone-resorbing' giant polykaryons that differentiate from mononuclear macrophage/monocyte-lineage hematopoietic precursors. However how the activity of mature osteoclasts is regulated in vivo remains unclear. To answer the question, we utilized an advanced imaging system for visualizing live bone tissues with intravital multiphoton microscopy that we have recently established. By means of the system we have recently succeeded in visualization of mature osteoclasts in live bones and revealed that RANKL regulates bone-resorptive functions of mature osteoclasts in vivo . Here we show the latest data and the detailed methodology of intravital imaging of bone tissues, and also discuss its further application.