Ischemia is the most common and important cause of injury to cardiomyocytes. Acute ischemia causes profound derangement of the cellular energetics and metabolism, and this ultimately leads to cell death. Experimental studies have demonstrated the presence of an endogenous protective mechanism that can diminish or delay cell death from ischemic insult; this is known as ischemic preconditioning. In this review, we summarize the recent knowledge of the cellular biology of acute ischemic injury and also signaling mechanisms of cardioprotection that are involved in preconditioning. Further, we briefly discuss the clinical implications.