Forgetting occurs when previously stored information cannot be retrieved. There are several theories for why forgetting occurs, including decay theory, interference theory, and retrieval failure theory. Decay theory proposes that memories fade over time as the memory traces in the brain weaken without reinforcement. Interference theory suggests that new learning can interfere with retrieving older memories. Retrieval failure theory posits that forgetting is due to an inability to access stored information using the appropriate retrieval cues, rather than the information actually being lost.