Cell injury, in a human body, provokes a variety of responses: vascular, inflammatory, and immune mediated, which occur via several independent or shared mechanisms. These responses may be slow or rapid and may occur simultaneously or in succession and may, at times, also extend beyond the boundaries of the injured cell. Cells constantly interact with their surroundings, thereby adjusting both structurally and functionally to accommodate the changing environments or its demands. This occurs via homeostasis, a self-regulating process that ensures stability of the cell while still allowing it to adapt to stress. Cell injury disrupts homeostasis. Injured cell then may respond positively (e.g.,: increased efficiency or productivity) or negatively (degeneration, decreased functional capacity, etc.). Cell injury is thus the initial event during the occurrence of most diseases of the body. In this chapter, we try to explain the triggers of cell injury, the mechanisms by which they affect the cell and finally, the various responses of cells to the injury-causing stimulus.