The changes in diagnostic criteria for major depressive disorder (MDD) from the fourth to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM) may appear small but have important consequences for how the diagnosis is used. In DSM-5, MDD is part of the new “Depressive disorders” section, which is separate from “Bipolar disorders”, marking a division in what had been known as “Mood disorders”. A small wording change has expanded the core mood criterion to include hopelessness, potentially broadening the diagnosis. The replacement of an operationalized bereavement exclusion with a call for clinical judgment in distinguishing normal reactions to significant loss from a disorder in need of clinical attention makes the diagnosis less objective and complicates investigations of the relationship between adversity and depression. A new persistent depressive disorder category is intended to encompass both dysthymia and chronic depression, but its relationship to MDD is ambiguous with conflicting statements on whether the two diagnoses should be concurrent if both sets of criteria are fulfilled. Clarification is also needed on whether MDD can be concurrent with the new broad “other specified bipolar and related disorders”. New specifiers of MDD “with anxious distress” and “with mixed features” allow characterization of additional symptoms. The specifier “with perinatal onset” expands on the DSM-IV “postnatal onset” to include onset during pregnancy. We review the changes in MDD definition, provide guidance on their implementation and discuss their implications for clinical practice and research.