Perdurantism or perdurance theory is a philosophical theory of persistence and identity. In metaphysics the debate over persistence currently involves three competing theories—one three-dimensionalist theory called "endurantism" and two four-dimensionalist theories called "perdurantism" and "exdurantism". For a perdurantist, all objects are considered to be four-dimensional "worms" that make up the different regions of spacetime. It is a fusion of all the perdurant's instantaneous time slices compiled and blended into a complete mereological whole. Perdurantism posits that temporal parts alone are what ultimately change. Katherine Hawley in How Things Persist states that change is "the possession of different properties by different temporal parts of an object".

Take any perdurant and isolate a part of its spatial region. That isolated spatial part has a corresponding temporal part to match it. We can imagine an object, or four-dimensional worm: an apple. This object is not just spatially extended but temporally extended. The complete view of the apple includes its coming to be from the blossom, its development, and its final decay. Each of these stages is a temporal time slice of the apple, but by viewing an object as temporally extended, perdurantism views the object in its entirety.

The use of "endure" and "perdure" to distinguish two ways in which an object can be thought to persist can be traced to David Kellogg Lewis (1986). However, contemporary debate has demonstrated the difficulties in defining perdurantism (and also endurantism). For instance, the work of Ted Sider (2001) has suggested that even enduring objects can have temporal parts, and it is more accurate to define perdurantism as being the claim that objects have a temporal part at every instant that they exist. Currently, there is no universally acknowledged definition of perdurantism. Others argue that this problem is avoided by creating time as a continuous function, rather than a discrete one.

Perdurantism is also referred to as "four-dimensionalism" (by Ted Sider, in particular), but perdurantism also applies if one believes there are temporal but non-spatial abstract entities (like immaterial souls or universals of the sort accepted by David Malet Armstrong).

Four-dimensionalist theorists break into two distinct sub-groups: worm theorists and stage theorists.

Worm theorists believe that a persisting object is composed of the various temporal parts that it has. It can be said that objects that persist are extended through the time dimension of the block universe much as physical objects are extended in space. Thus, they believe that all persisting objects are four-dimensional "worms" that stretch across space-time, and that you are mistaken in believing that chairs, mountains, and people are simply three-dimensional.

Stage theorists take discussion of persisting objects to be talk of a particular temporal part, or stage, of an object at any given time. So, in a manner of speaking, a subject only exists for an instantaneous period of time. However, there are other temporal parts at other times which that subject is related to in a certain way (Sider talks of "modal counterpart relations", whilst Hawley talks of "non-Humean relations") such that when someone says that they were a child, or that they will be an elderly person, these things are true, because they bear a special "identity-like" relation to a temporal part that is a child (that exists in the past) or a temporal part that is an elderly person (that exists in the future). Stage theorists are sometimes called "exdurantists".

Exdurantism, like perdurantism, presumes the temporal ontology of eternalism. With this alternative four-dimensionalist persistence theory, however, ordinary objects are no longer perduring worms but, rather, are wholly present instantaneous stages. Moreover, things also do not gain or lose properties/parts because each distinct stage has all these properties/parts in their entirety from one counterpart stage to the next.