Action research provides valuable opportunities for preservice teachers to improve their practice, their understanding of their practice, and the situation in which their practice takes place. Moreover, action research empowers preservice teachers to critically examine an experience that demonstrates their potential to be influential researchers within their educational situations. In this study we utilize the construct of teacher uncertainty to characterize different dilemmas that preservice science and mathematics teachers encounter as they embark upon their first action research experience. Our study is guided by the following questions: what types of uncertainty do preservice teachers encounter; and in what ways can teacher action research facilitate how preservice teachers confront and embrace their uncertainties with becoming teachers as well as teacher researchers? This study represents a collaboration between a science teacher educator from the United States and a mathematics teacher educator from Ireland. We merge our research methodologies including data from qualitative sources (i.e. semi-structured interviews, reflections, and final action research papers) to isolate trends among the teachers' uncertainties. Results indicated that preservice teachers harbour a range of uncertainties not only with learning how to teach science and mathematics but also with learning to engage in action research. A case can be made that doubts about one's efficacy as a science or mathematics teacher are sometimes difficult; however, through action research, this sense of uncertainty can be recognized, accepted, and addressed in a positive and productive manner.