Investment decisions play a crucial role in the success of individuals, businesses, and economies. Understanding the factors that influence these decisions is essential for investors, financial advisors, and policymakers. This article provides a comprehensive review of the key factors affecting investment decisions, drawing from a wide range of academic literature and empirical studies. The article begins with the importance of psychological factors in investment decisions. It examines behavioral biases, including anchoring, loss aversion, and herding behavior, and their impact on investment choices. Additionally, the article explores the influence of external factors on investment decisions. It examines macroeconomic factors, including interest rates, inflation, and fiscal policies, and how they shape investment behavior. It also considers regulatory factors, market structure, and technological advancements as important external determinants. The article addresses the impact of investor demographics on investment decisions. It investigates how factors like age, income, education, and financial literacy influence risk preferences, investment goals, and asset allocation choices. By providing a comprehensive review of these factors, this article contributes to a deeper understanding of the complex dynamics underlying investment decisions. It offers valuable insights for investors, financial professionals, and policymakers, enabling them to make informed decisions and design effective strategies in the dynamic world of investing.