Engineering professors are well positioned to support their undergraduate students, who often experience diminished mental health. This paper examines engineering professors' perceptions of their undergraduate engineering students' experiences of stress. The described perceptions include when they notice student stress, which stressors they perceive, and supports that these students use. In this qualitative study, we interviewed 24 engineering professors and four career advisors at 18 institutions in the United States about these topics. Results show that these professors often had consistent access to notice indicators of student distress. They described key sources of student stress: balancing responsibilities, significant academic stress, and a culture of competition. They were less likely to notice student stressors associated with interpersonal relationships and identity-related stressors, which are less related to their role as professors. Supports that professors described included interpersonal relationships and health and wellness activities. This lays a foundation for encouraging engineering professors to support their students' mental health and wellness even more.