This multi-method inquiry probes the experience and effectiveness of consumer-generated advertising [...](CGA) and explores four mechanisms that might explain their effects: source credibility, source identification, engagement and quality of the ad. An online experiment provides a generalizable test of CGA performance and shows that (1) CGAs generate more favorable ad responses than company ads; (2) CGA advantages derive partly from source credibility effects, but credibility advantages do not fully account for effectiveness; (3) Identification with the source does not drive performance; and (4) Increases in cognitive, emotional, personal and behavioral engagement distinguish CGA from company ads. A second experiment replicates response advantages for CGAs and provides evidence of the interactive role that ad quality plays. A third study delves into the engagement mechanism within a natural viewing environment by providing a content analysis of viewer responses to a range of different CGAs. Social engagement emerges as key facet of the CGA experience and the ad creator is identified as a key stakeholder in this process. CGAs do not present consistent advantages across categories and settings, suggesting a contingency-based understanding of CGA effects.