Forgetfulness is a common part of life and it ranges from the least impacting occasions, such as not bringing the grocery list to the supermarket, to much more severe situations, such as forgetting to write an essay the evening before it was due. The reason for this malfunction of information retrieval can be sorted into two categories of answer. The first answer, often related to the Short Term Memory or STM, is that the memory has fled and is no longer retrievable. To explain this phenomenon, many relate short term memory lapse to displacement of memory, or to the decay of the memory’s traces in the brain. The second, often related to the Long Term Memory or LTM, is that the memory is stored in the nervous system but cannot be accessed. Often cases such as these will be attributed to the interference of other information, lack of consolidation of a memory, or the failure to retrieve memories. (McLeod, 2008) In this essay we will discuss the theories behind forgetfulness in both the STM and LTM. One concept prevalent throughout the study of memory lapse in the STM is that of trace decay. This theory assumes that memories leave distinct chemical or physical traces throughout the nervous system. In the STM, information is only held for fifteen to thirty seconds unless it is rehearsed or memorised. Forgetting occurs because of automatic decay or the fading of the memory’s trace. This theory is purely concerned with time and the limited span of the STM and does not concern itself with distracting activities that could take place in the mean time. (McLeod, 2008) The Brown-Peterson task was created in order to test this idea. in this study, the experimenter said 3 letters and a number, then the participants were asked to count backwards by threes in order to prevent rehearsal and memorisation of the information. After a certain period of time, participants were then asked to recall the three letters. This experiment was performed at three, six, nine, twelve, fifteen, and eighteen second intervals and it was found that after three seconds, participants were able to recall approximately 80% of the letter combinations, but after eighteen seconds, they could only recall 10% (Peterson, L.R., & Peterson, M.J., 1959). This shows that STM has a limited duration if memorization is prevented. Another theory often attributed to forgetfulness in the STM is that of Displacement. This theory assumes that when STM is full, newest information pushes out the oldest, subsequently displacing it from memory (McLeod, 2008). This theory is often tested through the “Free Recall Method,” the findings of which are generally reliable as they tend to turn out similar results in each trial. (Murdock, 1962) In Atkinson and Shiffrin’s 1968 experiment, participants were asked to listen to an entire list of words b