<p>Fear is a fundamental emotion that transcends individual psychology to influence complex systems across ecology, health, technology, and economics. While often treated as a peripheral issue or a simple psychological outcome, its role as a primary, quantifiable driver of decisions is frequently neglected in systemic modeling. This review aims to address this gap by clearly identifying how fear operates as a dynamic, measurable force across disciplines, and by outlining the key mechanisms through which it reshapes system behaviour. It systematically explores how fear acts as a dynamic driver of behavior and system outcomes in four major domains-population dynamics, medical science, cybercrime, and financial markets over the past three decades of research. The study is motivated by the growing recognition that fear operates not only as an emotional state but also as a quantifiable factor shaping stability, resilience, and feedback mechanisms in natural and man-made systems. To establish the significance of the work, we articulate the need for an integrated, cross-disciplinary understanding of fear and highlight how existing literature in each domain remains fragmented. By critically synthesizing theoretical models, empirical studies, and computational approaches, we map how fear modifies reproduction and interaction in ecological populations, compliance and risk perception in medical contexts, manipulation and victim response in cyberspace, and investor sentiment and volatility in financial markets. In population ecology, fear- particularly that induced by predation-modifies prey behavior, reduces reproduction rates, and alters population trajectories, thereby acting as a non-consumptive force with ecosystem-level consequences. In this case, this review encompasses a broad spectrum of modeling approaches, including temporal, delayed, spatio-temporal, fractional, and stochastic frameworks. Both specialist and generalist predator–prey interactions have been considered to capture diverse ecological dynamics from a modeling perspective. The analysis primarily focuses on studies published between 2016 and 2025, providing a comprehensive exploration of the impact of fear on system behavior in population dynamics. This explicit time frame establishes the scope of the ecological literature reviewed, ensuring methodological clarity and transparency. In medical science, fear manifests both as a symptom and a catalyst, impacting disease progression, treatment compliance, vaccine hesitancy, and patient outcomes, especially in the context of pandemics and chronic illnesses. The realm of cybercrime reveals fear as both a psychological weapon and an outcome - phishing, online harassment, and cyberterrorism exploit fear to manipulate targets, while victims suffer long-term emotional and cognitive effects. In financial markets, fear drives investor sentiment, fuels speculation, and contributes to bubbles, crashes, and volatility, often amplified by media and algorithmic trading. By integrating theoretical models, empirical studies, and real-world observations, this review highlights fear as a central yet understudied driver of system dynamics, advocating for cross-disciplinary frameworks to better understand, predict, and manage its influence in diverse fields. It also argues that fear is not merely a reaction but a critical and predictive mechanism that can be quantified and modeled. We demonstrate that elevating fear from a neglected factor to a major explanatory variable is essential for building more robust and realistic models of complex systems. Overall, the review contributes a unified conceptual and methodological framework, establishes clear research gaps, and positions fear as a foundational parameter for future interdisciplinary modeling efforts.</p>