Background: While attrition from the PhD has been attributed to many high-level causal factors, such as funding, advisor relationship, and "fit" into a department, few studies have closely examined the mechanisms of attrition or why and how graduate engineering students begin to consider attrition from their doctoral programs.Design/Method: This study analyzed interviews with current and former doctoral engineering students at research universities across the United States, collected through two closely-related studies on graduate engineering experiences and attrition consideration. We used critical event analysis as a methodological approach to understand the experiences of a subset of 13 participants, who, at some point in their graduate career, experienced a singular event that caused them to question whether to persist in their PhD program.Purpose/Hypothesis: The purpose of the present paper is to investigate how graduate engineering students begin to question whether they should remain in their PhD programs of study.Results: We categorized the environments in which critical events occurred into four quadrants along the lines of University and Nonuniversity Settings and Routine versus Unexpected Contexts, mapping critical events and supporting events to themes from prior literature. The findings demonstrate how seemingly mundane experiences for faculty can be cataclysmic in the eyes of the student; how critical events serve to magnify other issues that had been accumulating over time; and how students may not self-reflect on their rationale for pursuing a PhD until a critical juncture occurs. Conclusions: Critical events are one mechanism by which students may begin considering departure from their engineering PhD programs. Some critical events masquerade within mundane contexts, like conversations or conferences (although, in retrospect, students can identify other relevant features contributing to dissatisfaction). From this work, we provide implications geared toward administrators, advisors, and graduate students on how to address and potentially mitigate critical events or their effects, including engaging in conversations about leaving.