Axiology is the philosophical study of value (from Greek, axia: "value, worth"; and -o, -logia: "study of"). It addresses issues such as the nature and classification of values, as well as the types of items that have value. It is intertwined with a number of other philosophical topics that rely heavily on the concept of value, such as ethics, aesthetics, and religious philosophy. Value theory and meta-ethics are also closely related. Paul Lapie coined the word in 1902, while Eduard von Hartmann coined it in 1908.

Axiology revolves around the distinction between intrinsic and extrinsic value. According to one interpretation, anything is intrinsically valued if it is valuable in and of itself. It is commonly assumed that intrinsic value is determined by certain characteristics of the valued entity. An experience, for example, may be argued to be intrinsically valued just because it is pleasurable. Extrinsic value, on the other hand, is given to things that are merely important as a route to another end. The goal of substantive value theories is to discover whether entities have intrinsic value. According to monist views, there is only one sort of intrinsic value. Hedonism, the belief that only pleasure has intrinsic value, is a classic example of monist ideas. Pluralist theories, on the other hand, argue that there are other sorts of intrinsic value, such as virtue, knowledge, and friendship. When it comes to making logical decisions, value pluralists must explain whether or how multiple categories of value can be compared. Values, according to some philosophers, do not exist at the most fundamental level of reality. According to one perspective, a value statement about anything simply indicates the speaker's acceptance or disapproval of it.