Noise, defined as random or irregular fluctuations that interfere with signals, is a fundamental issue in information processing and affects all aspects of nervous system function. The nature, amount, and impact of noise in the nervous system have been recently addressed quantitatively. This review discusses the sources of noise from the molecular to the behavioral level and how it contributes to trial-to-trial variability. It highlights how noise affects neuronal networks and the principles the nervous system uses to counter its detrimental effects, while also briefly discussing potential benefits of noise. Noise is present in sensory stimuli, cellular processes, and motor behavior, influencing perceptual thresholds and movement precision. The nervous system employs strategies such as averaging and prior knowledge to minimize the negative consequences of noise, which can also have beneficial effects on information processing. Understanding noise is crucial for comprehending the nervous system's structure and function, and its presence is inevitable due to the nanometer-scale operation of brains.Noise, defined as random or irregular fluctuations that interfere with signals, is a fundamental issue in information processing and affects all aspects of nervous system function. The nature, amount, and impact of noise in the nervous system have been recently addressed quantitatively. This review discusses the sources of noise from the molecular to the behavioral level and how it contributes to trial-to-trial variability. It highlights how noise affects neuronal networks and the principles the nervous system uses to counter its detrimental effects, while also briefly discussing potential benefits of noise. Noise is present in sensory stimuli, cellular processes, and motor behavior, influencing perceptual thresholds and movement precision. The nervous system employs strategies such as averaging and prior knowledge to minimize the negative consequences of noise, which can also have beneficial effects on information processing. Understanding noise is crucial for comprehending the nervous system's structure and function, and its presence is inevitable due to the nanometer-scale operation of brains.