Decay is a type of forgetting that occurs when memories fade over time. This does NOT apply to Long Term Memory, but rather sensory storage and Short Term Memory. The main reason this occurs in sensory and/or short term memory is that we don’t need to process and store all the information that we encounter in the world, so we simply don’t attend to, recognize, or rehearse all the information, and this information just fades away not to be stored in our long term memories.