Passive antibody therapy, utilizing plasma or serum, has been employed successfully to combat infectious diseases for over a century by transferring antibodies from recovered individuals to recipients, potentially mitigating disease progression. During the COVID-19 pandemic, convalescent plasma emerged as a promising therapeutic option, receiving emergency use authorization and widespread adoption. Its historical efficacy in previous pandemics, biological plausibility, and initial lack of targeted antivirals made it a cornerstone response. While initial studies suggested benefits, later research showed mixed results and faced challenges in determining its effectiveness. However, recent evidence indicates that when administered early and with high antibody levels, convalescent plasma can be effective in preventing disease progression and reducing mortality in COVID-19 patients. This chapter provides a comprehensive overview of the principles, and mechanisms of convalescent plasma therapy, summarizes current evidence for its use in COVID-19, and highlights its efficacy, importance, and associated challenges.