Processing fluency, or the subjective experience of ease with which people process information, reliably influences people's judgments across a broad range of social dimensions. Experimenters have manipulated processing fluency using a vast array of techniques, which, despite their diversity, produce remarkably similar judgmental consequences. For example, people similarly judge stimuli that are semantically primed (conceptual fluency), visually clear (perceptual fluency), and phonologically simple (linguistic fluency) as more true than their less fluent counterparts. The authors offer the first comprehensive review of such mechanisms and their implications for judgment and decision making. Because every cognition falls along a continuum from effortless to demanding and generates a corresponding fluency experience, the authors argue that fluency is a ubiquitous metacognitive cue in reasoning and social judgment.