What is obsolescence? Numerous older housing blocks have been knocked down because of being obsolete. There is a general understanding that buildings, like machinery and durable consumer goods, should be replaced when they become obsolete. But is this true? Obsolescence is a serious threat for built property. Given its immobile, long lasting and capital intensive character and its societal and cultural significance on the one hand and the high uncertainty about their future lives on the other, minimizing obsolescence is indispensible for the up keeping of the physical, economical and societal investments involved. This article explores the characteristics and symptoms of obsolescence, how can they be diagnosed and when and to what extent is demolition an unavoidable consequence? Due to the limited availability of empirical sources, the approach is mainly inventory and conceptual, based on literature search supported by previous empirical work.