Influencer marketing is a marketing strategy in which brands collaborate with creators — known as influencers or KOL (Key Opinion Leader)s — who have established, engaged audiences to promote products or services. On YouTube, influencer marketing typically takes the form of sponsored content, dedicated review videos, product integrations, or brand collaborations. It is effective because influencer social proof carries more trust than traditional advertising: audiences follow creators for their authentic voice, making endorsements feel more credible. Campaign success is measured using metrics like reach, engagement rate, conversion rate, and ROI (Return on Investment).