In a world in which more and more fake news is being spread, it is becoming increasingly difficult to distinguish truth from lies, knowledge from opinion. Disinformation campaigns are not only perceived as a political problem, but the fake news debate is also about fundamental philosophical questions: What is truth? How can we recognize it? Is there such a thing as objective facts or is everything socially constructed? This book explains how echo chambers and alternative worldviews emerge, it blames post-factual thinking for the current truth crisis, and it shows how we can escape the threat of truth relativism.