Wrinkles are linear depressions in the skin that appear over time. Aging due to the passage of time (crono-aging), develops with reabsorption of bone tissue, reabsorption and ptosis of adipose tissue, and increased laxity of all the tissues which tend therefore to slide downwards. Photo-aging, with different degrees of severity, adds to the effects of crono-aging; constant exposure to UV rays causes direct damage to the cellular DNA of the skin, production of free radicals that can damage cell membranes, and activation of enzymes (MMP or metalloproteinases) that break down the collagen in the skin. Photo-induced skin damage appears as progressive development of dyschromia (lentigo), thin wrinkles, actinic keratosis and finally skin tumours. The combination of photo-aging and crono-aging causes the formation of wrinkles which can be classified, according to the Glogau wrinkle scale, as present only during movements of facial expression or present also at rest. The former, also called dynamic wrinkles, appear early and are caused by the constant, repeated contractions of the facial muscles which are directly connected to the skin and cause it to wrinkle. The latter, present also at rest and known as static wrinkles, originate with all the aging processes described above. A hollow, on the other hand, is a depression in the skin already present in early age, and deepened by laxity and ptosis of the skin later on. While dynamic wrinkles (forehead, glabella, periocular) can be made less obvious with botulinum toxin, static wrinkles can be improved with fillers or laser resurfacing. While hollows can be improved with fillers or surgically by fat transfer, the most severe cases of static wrinkles require a purely surgical approach (face-lift).