This book presents up-to-date information on the clinical-pathophysiological features of acute renal injury and discusses the KDIGO diagnostic criteria, as well as novel experimental findings, including in the area of regenerative medicine. It also highlights the clinical-pathophysiological importance of AKI in clinical settings, including differential diagnoses and management of AKI.  In the past, the pathology associated with sudden renal impairment was characterized as acute renal failure (ARF). However, in the 2000s, the joint efforts of specialists in fields including nephrology, intensive care medicine, and cardiovascular medicine led to the introduction of a novel concept known as acute kidney injury (AKI). As medical care progressed, patients such as high-risk elderly subjects who were not deemed to be candidates for invasive therapy came to be treated in intensive care units (ICUs). As a result, kidney injury as a subset of multiple organ failure was re-considered as AKI, especially in intensive care medicine. AKI was then proposed as a novel disease concept to emphasize the importance of early diagnosis and early intervention to improve prognosis. Presenting novel features, such as the definition of AKI, risk factors and management; biomarkers, such as neutrophil gelatinase-associated lipocalin (NGAL) and L-type fatty acid-binding protein (L-FABP); long-term outcomes of AKI; as well as renal regeneration using iPS cell, manipulation of embryonic genes, and Xenotransplanted embryonic kidney, this book is of interest to all physicians and researchers in this field around the globe.