On May 2017, the World Health Organization (WHO) recognized sepsis as a global health priority by adopting a resolution to improve the prevention, diagnosis, and management of this deadly disease. While it has long been known that sepsis deeply perturbs immune homeostasis by inducing a tremendous systemic inflammatory response, pivotal observations based on clinical flow cytometry indicate that sepsis indeed initiates a more complex immune response that varies over time, with the concomitant occurrence of both pro-and anti-inflammatory mechanisms. As a resultant, some septic patients enter a stage of protracted immunosuppression. This paved the way for immunostimulation approaches in sepsis. Clinical flow cytometry permitted this evolution by drawing a new picture of pathophysiology and reshaping immune trajectories in patients. Additional information from cytometry by time of flight mass cytometry and other highdimensional flow cytometry platform should rapidly enrich our understanding of this complex disease. This review reports on landmarks of clinical flow cytometry in sepsis and how this single-cell analysis technique permitted to breach the wall of decades of unfruitful anti-inflammatory-based clinical trials in sepsis.On May 2017, the World Health Organization (WHO) recognized sepsis as a global health priority by adopting a resolution to improve the prevention, diagnosis, and management of this deadly disease (1). Sepsis is defined as a life-threatening organ dysfunction caused by a dysregulated host response to infection (2). Septic shock is the most severe form of sepsis in which hypotension persists despite adequate volume resuscitation thus requiring the use of vasopressors. According to recent definitions, in response to an infectious trigger, the unbalanced host (immune) response is at the center of sepsis pathophysiology (2).Sepsis represents a major healthcare problem worldwide. As an example, in the US, sepsis is more frequent than myocardial infarction, breast, or colon cancers (3-6). Recent modeling of available data determined the number of cases of sepsis in the entire world to over 20-25 million corresponding to >6 million deaths per annum (7,8). Over the years, 28-day mortality has remained high, ranging from 20% for sepsis to over 40% for septic shock despite improvement in patients' care (fluid resuscitation, source control, antimicrobial therapy). Whereas few therapeutic options evaluated in phase II clinical trials have demonstrated the potential efficacy of immunostimulation in sepsis, to date, no therapeutic intervention targeting host response has specifically been approved and sepsis remains the leading cause of mortality in intensive care units (ICU)(9,10). In addition, sepsis incidence has dramatically increased over the past decade and is expected to further augment. This rising