Periodic boundary conditions (PBCs) are a set of boundary conditions that are often chosen for approximating a large (infinite) system by using a small part called a unit cell. PBCs are often used in computer simulations and mathematical models. The topology of two-dimensional PBC is equal to that of a world map in some video games; the unit cell's geometry satisfies perfect two-dimensional tiling, and after an object passes through one side of the cell, it reappears on the opposite side with the same velocity. In topological terms, the space made by two-dimensional PBCs can be thought of as being mapped onto a torus (compactification). The large systems approximated by PBCs consist of an infinite number of unit cells. In computer simulations, one of these is the original simulation box, and the others are copies called images. During the simulation, only the properties of the original simulation box must be recorded and propagated. The minimum-image convention is a common form of PBC particle bookkeeping in which each particle in the simulation interacts with the closest image of the remaining particles.

One example of periodic boundary conditions can be defined according to smooth real functions ${\displaystyle \phi :\mathbb {R} ^{n}\to \mathbb {R} }$ by

for all m = 0, 1, 2, ... and for constants ${\displaystyle a_{i}}$ and ${\displaystyle b_{i}}$.

In molecular dynamics simulations and Monte Carlo molecular modeling, PBCs are usually applied to calculate properties of bulk gases, liquids, crystals, or mixtures. A common application uses PBC to simulate solvated macromolecules in a bath of explicit solvent. Born–von Karman boundary conditions are periodic boundary conditions for a special system.

In electromagnetics, PBC can be applied for different mesh types to analyze the electromagnetic properties of periodical structures.

Three-dimensional PBCs are useful for approximating the behavior of macro-scale systems of gases, liquids, and solids. Three-dimensional PBCs can also be used to simulate planar surfaces, in which case two-dimensional PBCs are often more suitable. Two-dimensional PBCs for planar surfaces are also called slab boundary conditions; in this case, PBCs are used for two Cartesian coordinates (e.g., x and y), and the third coordinate (z) extends to infinity.

PBCs can be used in conjunction with Ewald summation methods (e.g., the particle mesh Ewald method) to calculate electrostatic forces in the system. But PBCs also introduce correlational artifacts that do not respect the translational invariance of the system, requiring constraints on the composition and size of the simulation box.

In simulations of solid systems, the strain field arising from any inhomogeneity in the system will be artificially truncated and modified by the periodic boundary. Similarly, the wavelength of sound or shock waves and phonons in the system is limited by the box size.