Serum ionized calcium (iCa) is often measured in patients admitted to ICUs, and at least half of these patients will have values outside the reference range during their ICU stay. The vast majority of these patients do not have an underlying disease of calcium homeostasis. This Contemporary Review discusses the rationale for measurement of iCa and whether available data support its measurement and correction. It is determined that while measurement of serum iCa is commonplace and attempted correction is popular, available evidence and logical analysis do not wholly support these practices. Abnormal values of iCa are likely a marker of disease severity in critical illness and most often normalize spontaneously with resolution of the primary disease process. Alternatively, low iCa levels in critical illness may be protective and attempted correction of low levels may be harmful. Dramatic curtailment of iCa measurement and calcium administration in several studies was not associated with worsening outcomes. The absence of high-quality data to guide practice allows for a spectrum of approaches to the measurement and treatment of iCa, but these approaches should be guided by basic principles of rational clinical decision-making. Widespread, protocolized measurement and administration with the simple goal of normalizing values in the name of “euboxia” should be discouraged.