Service time in queuing theory is usually assumed to follow Exponential law.

Service time resembles interarrival time in that it is a continuous random variable, and it is also assumed to follow a probability distribution known as the negative exponential distribution. In analyzing service times, service must also be expressed as a rate (e.g, units per hour or per month) to be consistent with arrivals, which are also expressed as a rate.

The queue discipline is the method by which a part or customer is selected from a queue for service.

The service process is typically described by a probability distribution. The most commonly used distribution is the exponential distribution. As with the arrival process, the service process is assumed to be independent of the number of parts already in the system.

The exponential distribution is "memoryless. If the interarrival time distribution is exponential, it means that the time until the next arrival is independent of how long it has been since the last arrival. Thus, it is completely random. This property of the exponential distribution is also referred to as the no-memory, Markovian, or forgetfulness property.