 Healthy biological systems exhibit complex patterns of variability that can be described by mathematical chaos. These patterns are seen in the heart rate variability, HRV, of a healthy heart, which is not a metronome, but rather a constantly changing oscillation that allows the cardiovascular system to quickly adapt to sudden physical and psychological challenges to homeostasis. Current research has identified several different types of HRV measures, including time domain, frequency domain, and non-linear metrics. Time domain indices measure the amount of HRV observed during monitoring periods ranging from 2 minutes to 24 hours. Frequency domain values calculate the absolute or relative amount of signal energy within component bands. Non-linear measurements quantify the unpredictability and complexity of a series of interbeet intervals. Normative values for clinical, healthy, and optimal performance populations have been established, though it is important to consider the recording period length, subject age, and gender when interpreting these values. Additionally, professionals should supplement published norms with findings from their own specialized populations. Finally, HRV assessments can be used to inform clinical and optimal performance interventions. This article was authored by Fred Schaefer and JP Ginsberg. We are article.tv, links in the description below.