Active particles, unlike passive Brownian particles, can take energy from their environment and convert it into directed motion. This behavior is governed by nonequilibrium physics and is observed in biological systems such as cells, which move to find nutrients or avoid toxins. Inspired by these natural systems, researchers have developed artificial micro- and nanomachines that mimic self-propelled motion. These particles have potential applications in healthcare, sustainability, and security. This review explores the physical principles of active particles in complex environments, their behavior, and the challenges in their study. Active particles exhibit directed motion due to a combination of random fluctuations and self-propulsion. Their motion can be modeled using stochastic differential equations, which account for both translational and rotational diffusion. The effective diffusion coefficient and effective temperature are key parameters in understanding active particle behavior. Biological microswimmers, such as bacteria and sperm cells, exhibit various swimming patterns, including circular and helical motion, which can be explained by chiral active Brownian motion. Artificial microswimmers, such as Janus particles and chiral colloids, have been developed to replicate these behaviors. Active particles in homogeneous environments show distinct motion patterns compared to passive Brownian particles. Their trajectories are influenced by factors such as rotational diffusion, propulsion speed, and the presence of external forces. In complex environments, active particles interact with obstacles, walls, and other particles, leading to phenomena such as clustering, self-jamming, and active turbulence. These interactions are crucial for understanding the behavior of active matter in real-world conditions. The study of active particles is important for understanding nonequilibrium phenomena and developing new technologies. Challenges remain in scaling down active matter systems to the nanoscale and understanding their behavior in crowded environments. Future research will focus on improving models of active particle interactions, developing new propulsion mechanisms, and exploring applications in healthcare and environmental science. This review provides a comprehensive overview of active particles, their behavior, and the current state of research in the field.Active particles, unlike passive Brownian particles, can take energy from their environment and convert it into directed motion. This behavior is governed by nonequilibrium physics and is observed in biological systems such as cells, which move to find nutrients or avoid toxins. Inspired by these natural systems, researchers have developed artificial micro- and nanomachines that mimic self-propelled motion. These particles have potential applications in healthcare, sustainability, and security. This review explores the physical principles of active particles in complex environments, their behavior, and the challenges in their study. Active particles exhibit directed motion due to a combination of random fluctuations and self-propulsion. Their motion can be modeled using stochastic differential equations, which account for both translational and rotational diffusion. The effective diffusion coefficient and effective temperature are key parameters in understanding active particle behavior. Biological microswimmers, such as bacteria and sperm cells, exhibit various swimming patterns, including circular and helical motion, which can be explained by chiral active Brownian motion. Artificial microswimmers, such as Janus particles and chiral colloids, have been developed to replicate these behaviors. Active particles in homogeneous environments show distinct motion patterns compared to passive Brownian particles. Their trajectories are influenced by factors such as rotational diffusion, propulsion speed, and the presence of external forces. In complex environments, active particles interact with obstacles, walls, and other particles, leading to phenomena such as clustering, self-jamming, and active turbulence. These interactions are crucial for understanding the behavior of active matter in real-world conditions. The study of active particles is important for understanding nonequilibrium phenomena and developing new technologies. Challenges remain in scaling down active matter systems to the nanoscale and understanding their behavior in crowded environments. Future research will focus on improving models of active particle interactions, developing new propulsion mechanisms, and exploring applications in healthcare and environmental science. This review provides a comprehensive overview of active particles, their behavior, and the current state of research in the field.