This chapter examines the psychological foundations of misinformation, exploring how individuals come to believe and propagate false information. Drawing from cognitive and social psychology, it outlines the historical and contemporary dynamics of information manipulation through propaganda, media distortion, fake news, and conspiracy theories. The chapter details cognitive biases, motivated reasoning, and identity-related processes that sustain belief in falsehoods, despite corrections. Finally, it evaluates preventive and corrective interventions such as prebunking and debunking, and discusses their effectiveness in promoting resistance to misinformation in democratic societies.