Aging occurs with cell beginning to lose function in a variety of cell types. If it takes place in a given tissue or organ prematurely ahead of other tissues and organs, it causes pathological development of diseases. How to treat aging cells and associated molecules is a challenge in anti-aging research. It has been demonstrated recently that aging occurs under physiological conditions during embryonic development of mammals, and that to some life forms aging never occurs. Involved in diverse diseases, aging is difficult to be measured and does not have universal markers. It is thus essential to comprehend the types of cells that undergo aging in human diseases and thereby the underlying molecular mechanisms. This article discusses recent research advances including: (1) The concept, classification and relevant mechanisms of cell aging; (2) Physiological aging: Developmentally programmed senescence; (3) Tissue homeostasis and aging; (4) Cell replicative senescence and related diseases: telomeres and cancer prognosis, idiopathic pulmonary fibrosis, hypertension; (5) Non-replicative cell aging and related diseases: Parkinson disease and diabetes; (6) Species diversity in aging and longevity.