This paper presents affect as an essential aspect of students' self-reflection and self-regulation. The introduced concepts of self-system and self-system process stress the importance of self-appraisals of personal competence and agency in affective responses and self-regulation in problem solving. Students are viewed as agents who constantly interpret and evaluate their experiences and regulate their behaviour, in interaction with their mathematics learning environment. This perspective is used to interpret two data sets: Finnish secondary school students' questionnaire responses and their mathematics achievement, and Frank's problem solving episode. The former study examines statistical relations between self-confidence, positive and negative affective responses, self-regulatory patterns, and math performance. The latter focuses on Frank's appraisals and self-regulatory processes with his affective responses while problem solving. Finally, I consider the perspective's usefulness and compare it others in this Special Issue.