The overall aim of the dissertation is to deepen the understanding of how teachers, both individually and in groups, make didactic choices regarding the use of digital learning resources (DLR) in teaching, as well as the internal and external factors that influence these choices. The dissertation is grounded in didactic theory and is based on a combined methodological analysis, where a life history analysis is complemented by two survey studies with statistical analyses. Teachers' didactic choices of DLR are influenced by the social and historical context, where both individual and collective aspects play a role and include relationships, differences, and influencing factors. The life history analysis contributes a personal perspective and describes how upper secondary school teachers' didactic motives, considerations, and choices are shaped and developed over time. The first survey study characterizes new, unique DLR-teacher profiles for upper secondary school teachers, focusing on the didactic use of DLR. The study shows that a teaching degree is important for didactic practice with DLR. The second survey study examines the factors that influence teachers' use of DLR. The results indicate that a favorable school environment, social support, previous work experience, education, and personal interests are all important factors, but that student-oriented factors have the greatest impact. Many teachers use DLR in different ways as didactic support in teaching, but a smaller group experiences difficulties in managing DLR, which can cause stress. In summary, the dissertation provides new insights and a deeper understanding of the teacher's role as a reflective practitioner who makes meaningful didactic choices regarding DLR and the factors that influence these choices. This deeper understanding highlights that teachers' purposes, digital didactic capacity, subject interest, and willingness to help students are crucial for their didactic choices. The findings are highly relevant for teachers, teacher educators, principals, school leaders, and policymakers, as they can help improve the conditions for teachers' instructional practices and students' learning in a digitalized school environment.