Empirical investigations concerning the interplay between supervisor support and supervisor undermining behaviors and their effects on employees yielded contradictory findings, with some studies suggesting that support buffers the adverse effects of undermining, and others suggesting that support exacerbates these adverse effects. Seeking to explain such contradictory findings, we integrate uncertainty-management perspectives with coping theory to posit that relational uncertainty is inherent in the mixture of supervisor support and undermining. Hence, whether supervisor support buffers or exacerbates the adverse effects of supervisor undermining on employee health and well-being depends on factors pertaining to employee ability to resolve and manage such relational uncertainty. Specifically, we hypothesize a buffering effect for employees with high self-esteem and high quality of work life, and an exacerbating effect for employees with low self-esteem and low quality of work life. Analyses of 2-wave data collected from a probability stratified sample of U.S. Air Force personnel supported our predictions. Two supplementary studies of the U.S. military replicated our core findings and demonstrated its practical significance.