This chapter (1) shows how invasive alien species (IAS) have been defined and categorised, answering the question of what invasive alien species are and (2) discusses how this demarcation of a field of scholarship separate from the rest of the life sciences continues to be contentious. It suggests a continued need to understand how a distinct field of expertise progressively called ‘invasion science' has been constructed historically. It plots how this emerged and how it has been maintained as a bounded discipline by a specific cohort of scholars, examining how new individuals are enrolled in maintaining it and how power relations within this collective structure debates. By unpicking recent heartfelt scholarly debates around loaded terms such as colonisation and denialism, it seeks to make sense of the remaining contentious aspects of dissent and discord, in particular focusing on the role of humans as agents of ecological change, to unpack the remaining misunderstandings in a field where accusations of xenophobia and racism have clouded debates, rattled proponents and narrowed the possibility of constructive engagement.