A **constraint** is a limitation of possibilities. In particular, a constraint is a restriction of the feasible solutions in an optimization problem.

In the theory of constraints, it is any factor that limits the performance of a system with respect to its goal.

To take a simple example: a chain has 5 links, each link capable of holding a maximum weight of 5, 7, 4, 8 and 6 tonnes respectively. The maximum weight the chain can hold is clearly 4 tonnes – the limit imposed by the weakest link. In this case, all 5 links have *limits* to their strength, but the 3rd link is the *constraint* because it is the greatest restriction on the system.

Eliyahu M. Goldratt maintains that any business system has

- at least one constraint, otherwise its performance would be infinite
- very few constraints, otherwise it would be unstable and cease to exist

- list of theory of constraints topics
- restraint
- Donella Meadows' twelve leverage points to intervene in a system

For constraints in literature, see constrained writing and in theatre, see Theatrical constraints.