This article provides an overview of heart rate variability (HRV), a measure of the fluctuations in time intervals between consecutive heartbeats. HRV reflects the complex and dynamic interactions between the sympathetic and parasympathetic nervous systems, which regulate cardiovascular function. The authors discuss the importance of HRV in understanding health and performance, and review various metrics used to assess HRV, including time-domain, frequency-domain, and non-linear measures. They emphasize the context-dependent nature of HRV measurements, highlighting the impact of factors such as recording period length, subject age, and sex on baseline HRV values. The article also surveys published normative values for different populations and encourages professionals to consider their own specialized populations when interpreting HRV data. Finally, the authors provide strategies for assessing HRV in clinical and optimal performance interventions.This article provides an overview of heart rate variability (HRV), a measure of the fluctuations in time intervals between consecutive heartbeats. HRV reflects the complex and dynamic interactions between the sympathetic and parasympathetic nervous systems, which regulate cardiovascular function. The authors discuss the importance of HRV in understanding health and performance, and review various metrics used to assess HRV, including time-domain, frequency-domain, and non-linear measures. They emphasize the context-dependent nature of HRV measurements, highlighting the impact of factors such as recording period length, subject age, and sex on baseline HRV values. The article also surveys published normative values for different populations and encourages professionals to consider their own specialized populations when interpreting HRV data. Finally, the authors provide strategies for assessing HRV in clinical and optimal performance interventions.