There are many high-profile reports about the link between social media and mental health, and these sit within a broader social climate where people are asking pressing questions about the extent to which social media can ’affect’ or ’impact’ individuals. It is natural to fear new and fast-growing technologies, but regressing to the media ’effects’ models that have for so long been discredited may not be helpful, or indeed accurate. One of the aims of this chapter is therefore to argue that social media are not one thing and to instead introduce several aspects of the phenomenon to readers, briefly tracing standout phases in their evolution, the characteristics that differentiate them from older media technologies, their (increasingly controversial) business and governance models, and finally their use and non-use among particular social groups.