Memory involves encoding, storing, and retrieving information. Encoding transforms information into codes that create a memory trace stored in memory. There are two main types of memory stores - short term memory which can hold 7 +/- 2 items for up to 30 seconds, and long term memory which can hold memories for minutes to years without limit. Baddeley's model of working memory involves an attentional control system and two subsystems to support visual and verbal information. Levels of processing theory suggests that deeper processing of information results in stronger, more durable memories than shallow processing.