This chapter explores how social psychology underpins health promotion and illness prevention, providing insights into how individual and societal health behaviors are shaped by social, cognitive, and environmental factors. The discipline of social psychology offers a vital lens for understanding health behavior, notably through foundational theories like the Health Belief Model (HBM), Theory of Planned Behaviour (TPB), Social Cognitive Theory (SCT), and the Transtheoretical Model (TTM). Each theory contributes unique perspectives on factors such as perceived control, social influence, and self-efficacy, forming a comprehensive framework for addressing health-related decisions and behaviors. Key elements such as social norms, peer influences, and social identity are critical to understanding health behaviors within groups, whether in community settings or among at-risk populations. Social psychological approaches help explain how social support systems influence behavior, both positively and negatively, and how communication strategies—like persuasion, message framing, and fear appeals—impact behavior change in public health campaigns. Furthermore, the rise of digital media introduces new dynamics in health promotion, highlighting the importance of addressing both misinformation and effective message dissemination across diverse audiences. This chapter also considers the unique barriers to health behavior change in marginalized groups, focusing on social identity and stigma associated with conditions like mental illness or HIV/AIDS. Practical applications of social psychology in health promotion are illustrated through real-world interventions, including community-based initiatives, digital health programs, and workplace wellness efforts. By integrating social psychological theory with public health practices, this chapter aims to provide a thorough understanding of how health behaviors can be influenced and sustained, promoting healthier lifestyles and reducing health disparities.