In information technology and computer science, a system is described as stateful if it is designed to remember preceding events or user interactions; the remembered information is called the state of the system.

The set of states a system can occupy is known as its state space. In a discrete system, the state space is countable and often finite. The system's internal behaviour or interaction with its environment consists of separately occurring individual actions or events, such as accepting input or producing output, that may or may not cause the system to change its state. Examples of such systems are digital logic circuits and components, automata and formal language, computer programs, and computers.

The output of a digital circuit or deterministic computer program at any time is completely determined by its current inputs and its state.

Digital logic circuits can be divided into two types: combinational logic, whose output signals are dependent only on its present input signals, and sequential logic, whose outputs are a function of both the current inputs and the past history of inputs. In sequential logic, information from past inputs is stored in electronic memory elements, such as flip-flops. The stored contents of these memory elements, at a given point in time, is collectively referred to as the circuit's state and contains all the information about the past to which the circuit has access.

Since each binary memory element, such as a flip-flop, has only two possible states, one or zero, and there is a finite number of memory elements, a digital circuit has only a certain finite number of possible states. If N is the number of binary memory elements in the circuit, the maximum number of states a circuit can have is 2^N.

Similarly, a computer program stores data in variables, which represent storage locations in the computer's memory. The contents of these memory locations, at any given point in the program's execution, are called the program's state.

A more specialized definition of state is used for computer programs that operate serially or sequentially on streams of data, such as parsers, firewalls, communication protocols and encryption. Serial programs operate on the incoming data characters or packets sequentially, one at a time. In some of these programs, information about previous data characters or packets received is stored in variables and used to affect the processing of the current character or packet. This is called a stateful protocol and the data carried over from the previous processing cycle is called the state. In others, the program has no information about the previous data stream and starts fresh with each data input; this is called a stateless protocol.

Imperative programming is a programming paradigm (way of designing a programming language) that describes computation in terms of the program state, and of the statements which change the program state. Changes of state are implicit, managed by the program runtime, so that a subroutine has visibility of the changes of state made by other parts of the program, known as side effects.