Walking speed (WS) is a robust and sensitive measure used to assess and monitor functional status and overall health across various populations. It has been designated as the "6th vital sign" due to its predictive capabilities and ease of administration. The article provides clinicians with a comprehensive reference on WS, including testing procedures, optimal distances, inclusion of acceleration/deceleration phases, instructions, and instrumentation. It also offers cut-off values and minimal detectable change values for specific populations and settings. WS is associated with a range of outcomes, such as response to rehabilitation, functional dependence, frailty, mobility disability, cognitive decline, falls, institutionalization, hospitalization, and mortality. The article recommends testing WS over distances of 5 to 10 meters, with acceleration phases of approximately 2.5 meters for self-selected speeds and 3.25 meters for maximal WS. Straight paths are preferred over turns to capture steady-state WS. Handheld stopwatches and marked walkways are recommended for timing, with automatic timers being a reliable alternative. The article emphasizes the importance of consistency in testing procedures to ensure accurate conclusions and highlights the value of WS in guiding clinical decision-making and interventions.Walking speed (WS) is a robust and sensitive measure used to assess and monitor functional status and overall health across various populations. It has been designated as the "6th vital sign" due to its predictive capabilities and ease of administration. The article provides clinicians with a comprehensive reference on WS, including testing procedures, optimal distances, inclusion of acceleration/deceleration phases, instructions, and instrumentation. It also offers cut-off values and minimal detectable change values for specific populations and settings. WS is associated with a range of outcomes, such as response to rehabilitation, functional dependence, frailty, mobility disability, cognitive decline, falls, institutionalization, hospitalization, and mortality. The article recommends testing WS over distances of 5 to 10 meters, with acceleration phases of approximately 2.5 meters for self-selected speeds and 3.25 meters for maximal WS. Straight paths are preferred over turns to capture steady-state WS. Handheld stopwatches and marked walkways are recommended for timing, with automatic timers being a reliable alternative. The article emphasizes the importance of consistency in testing procedures to ensure accurate conclusions and highlights the value of WS in guiding clinical decision-making and interventions.