In this paper I use a general philosophy of science perspective in looking at the problem of justifying action research. First I try to clarify the concept of justification, by contrasting it with the concept of validity, which seems to be used almost as a synonym in some parts of the literature. I discuss the need for taking a stand in relation to the questions of validity and justification also in action research. Using the examples of the battle between Galileo Galilei (1564-1642) and the Catholic Church in the seventeenth century and the battle between Aage Bødtker Sørensen, Rickard Hackman and Norges råd for anvendt samfunnsforskning on one side, and the action researchers at the Norwegian Work Research Institute on the other side in the mid-1990s as a point of departure, I inquire into the problem of justifying action research by comparing some different traditions and their ways of justifying themselves.