To hide the effects of interrupts, operating systems provide a conceptual model consisting of sequential processes running in parallel. Processes can be created and terminated dynamically. Each process has its own address space.

For some applications, it is useful to have multiple threads of control within a single process. These threads are scheduled independently and each one has its own stack, but all the threads in a process share a common address space. Threads can be implemented in user space or in the kernel.

Alternatively, high-throughput servers may opt for an event-driven model instead. Here, the server operates as a finite state machine that responds to events and interacts with the operating system using non-blocking system calls.

Processes can synchronize with one another using synchronization and interprocess communication primitives, for example, semaphores, monitors, or messages. These primitives are used to ensure that no two processes are ever in their critical regions at the same time, a situation that leads to chaos. A process can be running, runnable, or blocked and can change state when it or another process executes one of the interprocess communication primitives. Interthread communication is similar.

A great many scheduling algorithms have been studied. Some of these are primarily used for batch systems, such as shortest-job-first scheduling. Others are common in both batch systems and interactive systems. These algorithms include round robin, priority scheduling, multilevel queues, guaranteed scheduling, lottery scheduling, and fair-share scheduling. Some systems make a clean separation between the scheduling mechanism and the scheduling policy, which allows users to have control of the scheduling algorithm.