The dynamics and uncertainties of environmental, physical, and social demands (e.g., climate change, technological evolution, population growth, and market changes) have a significant effect on the performance of infrastructure systems. Dynamic external conditions can affect the performance of infrastructure systems because traditional designs are thought for long-term demands and assume stationarity in future behavior. This results in rigid and permanent features that may not perform optimally under variable conditions. Specifically, the system may be rendered inefficient from an operational point of view, might be more exposed to risks, and may even stop serving its purpose to society altogether. This situation can be reduced by implementing alternative design and management strategies that facilitate changes in the system as unplanned scenarios arise. One novel strategy is to incorporate the possibility of change by making systems more flexible from the outset. While some benefits of introducing flexibility have been identified and discussed in the literature, the concept is still far from being completely understood. The objective of this work is to address this knowledge gap by presenting an analysis of the concept of flexibility. The analysis includes a literature review concerning the most relevant aspects of flexibility: quantification, valuation, and management. Additionally, the adaptation process is deconstructed to identify the core components that must be present in any flexibility analysis. From this analysis, the main system attributes affected by flexibility are made explicit to formulate a novel flexibility metric. Finally, three numerical examples are presented and discussed to illustrate the application of the metric and highlight the relationship between flexibility and the value added to the system.