Concept-oriented programming (CoP) is a programming paradigm which uses concepts as the main programming construct instead of classes as defined in OOP. A concept consists of one object class and one reference class. The object class is precisely what is meant by classes in OOP. Its instances, called objects, are passed-by-reference.

What is new in CoP is the presence of the reference classes. Their instances, called references, are passed-by-value and are intended to indirectly represent objects. Thus concepts can be viewed a means for modelling references. The structure and functions of references define how objects are represented and accessed. Each concept has a parent concept specified using inclusion relation which generalizes class inheritance. One consequence of having concept inclusion is that references get layered structure.

Such references consisting of several segments are referred to as complex references. Thus variables in CoP contain hierarchical addresses of objects in a virtual address space. Developing the structure of such a space is one of the main concerns in the concept-oriented program design. Concepts generalize conventional classes as used in OOP. Concept with the empty reference class is equivalent to a class and class with an additional reference class attached to it is a concept.

Concepts in the concept-oriented program are used just as traditional classes when declaring variables, fields, parameters, return values and other elements where we need a type. Any concept has a parent or base concept which is specified using concept inclusion relation. The inclusion structure of concepts defines how objects in the program are represented and accessed by defining their format of references.