Numeracy, the ability to understand and use numerical information, is crucial for making informed health decisions. Despite the abundance of health-related numerical information, many people lack basic numeracy skills, which can distort risk perception, reduce medication compliance, and impair risk communication. Low numeracy is associated with susceptibility to biases and extraneous factors, affecting medical outcomes. Research on numeracy is often not grounded in empirically supported theories, limiting the development of effective interventions. This review outlines four theoretical approaches—psychophysical, computational, dual-process, and fuzzy trace theory—to understand numeracy and its implications for health decision-making. It highlights the importance of numeracy in health contexts, such as understanding risk, benefits, and treatment options, and discusses the challenges faced by vulnerable populations. National and international surveys show that many adults lack basic numeracy skills, with significant disparities among subgroups. Assessments of numeracy reveal that even educated individuals struggle with basic numerical concepts. Specific instruments, such as the Test of Functional Health Literacy in Adults (TOFHLA), are used to measure numeracy, but they have limitations in capturing all aspects of numeracy. The review emphasizes the need for theory-driven research to improve numeracy and enhance health outcomes.Numeracy, the ability to understand and use numerical information, is crucial for making informed health decisions. Despite the abundance of health-related numerical information, many people lack basic numeracy skills, which can distort risk perception, reduce medication compliance, and impair risk communication. Low numeracy is associated with susceptibility to biases and extraneous factors, affecting medical outcomes. Research on numeracy is often not grounded in empirically supported theories, limiting the development of effective interventions. This review outlines four theoretical approaches—psychophysical, computational, dual-process, and fuzzy trace theory—to understand numeracy and its implications for health decision-making. It highlights the importance of numeracy in health contexts, such as understanding risk, benefits, and treatment options, and discusses the challenges faced by vulnerable populations. National and international surveys show that many adults lack basic numeracy skills, with significant disparities among subgroups. Assessments of numeracy reveal that even educated individuals struggle with basic numerical concepts. Specific instruments, such as the Test of Functional Health Literacy in Adults (TOFHLA), are used to measure numeracy, but they have limitations in capturing all aspects of numeracy. The review emphasizes the need for theory-driven research to improve numeracy and enhance health outcomes.