Answer - Concurrency management is closely related to the Instance management in WCF but both are two different things. Instance management specifies how the service instances are created while Concurrency management specifies how many concurrent requests are handled by the service instances. By using concurrency, you can make your service instance thread-safe. By default, a per-call service instance is thread-safe since each request is served by a new service instance. A per-session service instance is not thread-safe since multiple requests of a client are served by a single service instance. Hence, it’s required concurrency management. A single service instance is not thread-safe since multiple requests of all clients are served by a single service instance. Hence, it’s required concurrency management.