Long-term memory (LTM) is the stage of the Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to short-term and working memory, which persist for only about 18 to 30 seconds. Long-term memory is commonly labelled as explicit memory (declarative), as well as episodic memory, semantic memory, autobiographical memory, and implicit memory (procedural memory).

The modal model, proposed in the 1960s, suggests that memory is stored first in sensory memory, then short-term memory, and finally long-term memory. Evidence for the existence of separate memory stores includes anterograde amnesia and distraction tasks. However, the unitary model argues that short-term memory is a temporary activation of long-term representations. Experiments involving continual distractor tasks provide further evidence for a unified store.

Some models propose separate systems for short-term and long-term memory. The dual-store memory model, Atkinson-Shiffrin memory model, and Baddeley's model of working memory are examples of such theories. Long-term memory encodes information semantically and consolidates through a process called synaptic consolidation. Sleep is considered an important factor in establishing well-organized long-term memories, as it promotes the consolidation of new memories. Research has shown that sleep affects the retention of both declarative and procedural memories.

The brain stores different types of memory in various regions. Long-term memory is divided into explicit memory and implicit memory. Explicit memory, or declarative memory, includes consciously available memories and is linked to the medial temporal lobe. It is divided into three major subdivisions: episodic memory (memory for specific events), semantic memory (knowledge about factual information), and autobiographical memory (knowledge about events and experiences from an individual's own life). Implicit memory, or procedural memory, refers to the use of objects or movements of the body, like riding a bicycle. It is presumed to be stored by the striatum and other parts of the basal ganglia.

Emotional memory involves both declarative and procedural memory processes, while working memory is important for long-term memory function. Memory disorders can occur due to traumatic brain injury or neurodegenerative diseases like Alzheimer's, dementia, Huntington's, multiple sclerosis, and Parkinson's disease. Long-term memory depends on the synthesis of new proteins, and cellular processes like protein kinase C, BDNF, and cAMP response element-binding protein (CREB) play crucial roles in maintaining long-term memories. DNA methylation and demethylation are also involved in the formation and storage of long-term memories. Alternative memory models, like the single-store memory model, propose a single memory store with associations among items and their contexts.