Background: The plasticity of the human physiological system and behavioural adjustments made it possible for the human species to adapt to a wide array of climatic zones. Today, we manipulate the thermal environment to our desire, rather than adapting to the natural habitat. This is true for most developed countries, where people are rarely exposed to the variation of outdoor conditions, as they spend the majority of time indoors. Due to climate change, heat waves (such as 2003, 2018, 2019) will be more common, exposing the people living in Western and Central Europe to heat more often – both outdoors and indoors. In this context, the question arises on how increasingly frequent, long, and intense heat waves will affect human physiology and health. Even though heat waves may lead to excess mortality, especially among vulnerable populations, a controlled ‘heat training’ has the potential to increase resilience and has been suggested to even improve metabolic and cardiovascular health. Although it is well-known that active, exercise-induced heat acclimation enhances human performance in the heat, data on passive (without exercise) and milder heat acclimation regimens is lacking. Therefore, we performed two passive heat acclimation (PHA) studies, assessing the effect of exposure to elevated ambient temperature on thermophysiology and cardiometabolic parameters in healthy young and middle-aged overweight individuals. Methods: Eleven young healthy men (YH, 24.6±2.7y, BMI:22.6±2.9kg/m2) and eleven middle-aged, overweight (MO, 65.7±4.9y, BMI:30.4±3.2kg/m2) men participated in the two separate studies. Both populations were acclimated to heat (YH:7d, ~33˚C; MO:10d, ~34˚C) for 4-6h/d. Before and after PHA, core temperature (Tcore) and mean arterial pressure were measured during a temperature ramp protocol. In MO only, whole-body insulin sensitivity was assessed with a 1-step hyperinsulinemic-euglycemic clamp before and after PHA. Substrate oxidation was measured using indirect calorimetry during the clamps, and blood samples were drawn to assess markers of metabolic health. Results: Mean Tcore decreased in both groups post-PHA (YH:∆-0.14±0.15˚C, P=0.026; MO:∆-0.19±0.26˚C, P=0.036), and mean arterial pressure decreased in both populations after heat acclimation, to a variable extent. In MO, PHA reduced basal (9.7±1.4 pre vs. 8.4±2.1umol/kg/min post-PHA, P=0.038) and insulin-stimulated (2.1±0.9 pre vs. 1.5±0.8umol/kg/min post-PHA, P=0.005) endogenous glucose production (EGP). Consistently, fasting plasma glucose (6.0±0.5 pre vs. 5.8±0.4mmol/L post-PHA, P=0.013) and insulin (97±55 pre vs. 84±49pmol/L post-PHA, P=0.026) concentrations decreased. Moreover, fat oxidation increased, and free-fatty acids, as well as cholesterol concentrations in plasma, decreased after PHA.  Conclusion: PHA induces distinct thermophysiological and cardiovascular adaptations in both young healthy and middle-aged overweight individuals, which indicate increased resilience to heat. In the middle-aged overweight men, PHA also improves glucose homeostasis and enhances fat metabolism. Our results show that humans of both younger and older age and different bodily constitutions readily adapt to heat. Strikingly, heat acclimation seems to have the potential to improve cardiometabolic health, but underlying mechanisms are not yet fully understood. Even though it is well-known that heat waves may entail fatalities in vulnerable populations, it is worthwhile to further explore how heat affects our metabolic health and how resilience to heat can be improved.