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We reviewed the historical development of diagnostic nomenclature and classification systems of mood disorders.A literature search in PubMed and Google Scholar was performed using multiple search terms. Also, the criteria and classification of various mood disorders were reviewed and compared across all editions of DSM. We also reviewed several books and the references of the found articles.This review describes the historical development of the concepts and diagnostic nomenclature of mood disorders, including the encompassing of most of the now major depressive disorder under the prior manic-depressive illness. Additionally, we examine how mood disorders have been developed, classified, and split into subcategories historically until the current classification. We observed that the modern nosology (DSM-5) leans a bit more toward a spectrum approach.The pendulum has swung a bit from splitting toward lumping. The current diagnostic system blurs some of the boundaries between bipolar and unipolar disorders, as in the case of changing nomenclature to “mixed features” in both types of illnesses. This is supported by many experts (and some studies) who advocate for the spectrum concept in mood at the phenotypic level. The spectrum concept is more supported by evidence and further examination driven by both unconfined clinical observations and biological anchor points and markers to scientifically examine the zones of rarity and boundaries between disorders. This would be more fruitful than the arbitrary DSM number of criteria or episode durations and the artificial separation of manic-depressive illness.