This essay investigates the relationship between a malfunctioning Default Mode Network (DMN) and the diagnosis of Major Depressive Disorder (MDD). A deeper understanding of how the DMN affects those brain processes which are implicated in MDD may offer new approaches to reduce the suffering of the very large number of MDD-afflicted patients. The MDD-DMN relationship has been investigated by studying scientific articles within the field of cognitive neuroscience and searching the articles for clues on how a malfunctioning DMN might correlate with the diagnosis of MDD. The essay concludes that there is much experimental evidence in support of there being a strong coupling between a malfunctioning DMN and the diagnosis of MDD.