Noise in genetic circuits plays a crucial role in cellular functions, enabling coordination of gene expression, probabilistic differentiation, and evolutionary adaptation. This review discusses how noise, arising from stochastic fluctuations in gene expression, influences biological processes across different scales. Noise can enhance regulatory mechanisms, such as coordinating gene expression across large regulons, and enables probabilistic differentiation strategies that allow cells to respond to environmental changes. It also facilitates evolutionary transitions by allowing cells to explore different states. Gene expression noise is generated by stochastic bursts of protein production and time-averaging effects. These fluctuations can be analyzed through the distribution of protein levels and the timescale of expression. Noise can propagate through gene circuits, influencing downstream gene expression and enabling regulatory interactions. For example, in yeast, noise in the nuclear localization of the transcription factor Crz1 allows for coordinated gene regulation in response to calcium signals. Noise also enables probabilistic differentiation, allowing cells to adopt different fates in response to environmental cues. This is seen in systems such as bacterial sporulation, where noise in the accumulation of regulatory molecules leads to variability in sporulation outcomes. In stem cells, noise contributes to the stochastic determination of cell fates, enabling the generation of diverse cell types during development. Noise is also important in evolutionary processes, as it can increase the range of phenotypes that can arise from a given genotype. This is evident in the evolution of quantitative traits and in qualitative transitions during development. For example, mutations in developmental pathways can lead to partially penetrant effects due to noise, allowing for variability in phenotypic outcomes. The review highlights the importance of understanding the sources and effects of noise in gene circuits, as well as the challenges and opportunities in studying noise-driven processes. Future research aims to elucidate the functional roles of noise in cellular regulation, differentiation, and evolution, and to develop strategies for controlling noise in synthetic and natural systems.Noise in genetic circuits plays a crucial role in cellular functions, enabling coordination of gene expression, probabilistic differentiation, and evolutionary adaptation. This review discusses how noise, arising from stochastic fluctuations in gene expression, influences biological processes across different scales. Noise can enhance regulatory mechanisms, such as coordinating gene expression across large regulons, and enables probabilistic differentiation strategies that allow cells to respond to environmental changes. It also facilitates evolutionary transitions by allowing cells to explore different states. Gene expression noise is generated by stochastic bursts of protein production and time-averaging effects. These fluctuations can be analyzed through the distribution of protein levels and the timescale of expression. Noise can propagate through gene circuits, influencing downstream gene expression and enabling regulatory interactions. For example, in yeast, noise in the nuclear localization of the transcription factor Crz1 allows for coordinated gene regulation in response to calcium signals. Noise also enables probabilistic differentiation, allowing cells to adopt different fates in response to environmental cues. This is seen in systems such as bacterial sporulation, where noise in the accumulation of regulatory molecules leads to variability in sporulation outcomes. In stem cells, noise contributes to the stochastic determination of cell fates, enabling the generation of diverse cell types during development. Noise is also important in evolutionary processes, as it can increase the range of phenotypes that can arise from a given genotype. This is evident in the evolution of quantitative traits and in qualitative transitions during development. For example, mutations in developmental pathways can lead to partially penetrant effects due to noise, allowing for variability in phenotypic outcomes. The review highlights the importance of understanding the sources and effects of noise in gene circuits, as well as the challenges and opportunities in studying noise-driven processes. Future research aims to elucidate the functional roles of noise in cellular regulation, differentiation, and evolution, and to develop strategies for controlling noise in synthetic and natural systems.