Mental health’ is a catch-all term used to describe a wide range of conditions – from emotional states such as worry about academic progress or distress due to relationship problems or social challenges such as poverty, through to psychosis and other severe mental illness. To start therefore, this chapter will outline the definitions of some common terms used to describe those disturbances of mental health that are usually called emotional or mood disorders, and what might define mood disorders that are severe enough to be considered an illness. It will review what can be said with some certainty about social media use and disorders of mood, in a field where there is in truth a great deal of uncertainty. It will consider the common causes of mood disorders in general and what features of social media use might therefore lead us to be concerned that in any association between social media use and mood disorder, the former explains the latter. It will conclude with pointers to the research that needs to be done if the situation is to be clarified.