Pain assessment is examined in the perspective of action science with the aim to discover espoused theories and theories-in-use for pain assessment. In action science there are 2 sets of theories of action: espouse theories and theories-in-use, which often exist in practice inconsistently with each other resulting in haphazard actions. Espoused theories of pain and pain assessment and a set of theories-in-use were revealed. Alignments between the espoused theories and the theories-in-use, and disparities between these theories were found in the pain assessment situations. The findings point to possible explanations regarding problems in pain assessment, and provide insights into our understanding of nursing practice especially in relation to pain assessment.