Gait variability refers to the stride-to-stride fluctuations in walking and is a key indicator of gait stability and fall risk. This article reviews recent studies on gait variability, highlighting its importance in assessing mobility, fall risk, and the effectiveness of therapeutic interventions. The study of gait variability offers a complementary approach to quantifying locomotion and its changes with aging and disease. Previous research suggests that measures of gait variability may be more closely related to falls than measures based on the mean values of other walking parameters. The article discusses various methods for collecting gait data, including unconstrained ambulatory data and the use of global positioning systems. It also presents a primer on analysis methods and a heuristic approach to summarizing variability measures. Studies on gait variability in animal models of neurodegenerative diseases and a mathematical model of human walking are described. These studies reveal complex, multifractal features of motor control patterns. Research indicates that gait variability is influenced by factors such as neural control, muscle function, and postural control. Gait instability measures predict falls in elderly individuals, suggesting that gait variability may serve as a sensitive and clinically relevant parameter for assessing mobility and fall risk. The relationship between gait variability and heart rate variability is also explored, highlighting the complex, fractal-like nature of both systems. The article emphasizes the importance of studying gait variability in understanding the regulation of stride-to-stride fluctuations and its implications for gait control and clinical applications. It also addresses outstanding issues, such as the need for standardized measures and the development of open-access databases to facilitate research and clinical use. Overall, the studies presented in this series provide new insights into the factors that regulate gait variability and pave the way for expanded research into the control of gait and its practical application in clinical settings.Gait variability refers to the stride-to-stride fluctuations in walking and is a key indicator of gait stability and fall risk. This article reviews recent studies on gait variability, highlighting its importance in assessing mobility, fall risk, and the effectiveness of therapeutic interventions. The study of gait variability offers a complementary approach to quantifying locomotion and its changes with aging and disease. Previous research suggests that measures of gait variability may be more closely related to falls than measures based on the mean values of other walking parameters. The article discusses various methods for collecting gait data, including unconstrained ambulatory data and the use of global positioning systems. It also presents a primer on analysis methods and a heuristic approach to summarizing variability measures. Studies on gait variability in animal models of neurodegenerative diseases and a mathematical model of human walking are described. These studies reveal complex, multifractal features of motor control patterns. Research indicates that gait variability is influenced by factors such as neural control, muscle function, and postural control. Gait instability measures predict falls in elderly individuals, suggesting that gait variability may serve as a sensitive and clinically relevant parameter for assessing mobility and fall risk. The relationship between gait variability and heart rate variability is also explored, highlighting the complex, fractal-like nature of both systems. The article emphasizes the importance of studying gait variability in understanding the regulation of stride-to-stride fluctuations and its implications for gait control and clinical applications. It also addresses outstanding issues, such as the need for standardized measures and the development of open-access databases to facilitate research and clinical use. Overall, the studies presented in this series provide new insights into the factors that regulate gait variability and pave the way for expanded research into the control of gait and its practical application in clinical settings.