Science fiction’s most frequent alternative to human is 'alien', another rich imaginative resource with which to think about what makes us human. This lecture will include reflections on various aliens, from H.G. Wells Selenites, to Octavia Butler’s Oankali, the genetic traders who link the novels of her Xenogenesis trilogy, who are imagined as both saving and enslaving humanity. Whether aliens are imagined as conquerors or saviours, their superiority has often been used to explore human limitations.