Justification is understood as the task of convincing users that the output of a software system is correct and useful, providing sound specific support and credible reasons. This paper discusses the role of justification in making complex software systems more usable and acceptable, and proposes a novel approach to the design of justification systems. A generic, highly reusable architecture is defined and commented. It includes causal, teleological and behavioral models of the target application domain, a knowledge-base containing individual user profiles, and a set of reasoning mechanisms, based on general justification principles. The proposed approach, which supports a broad class of justifications, has been tested in a case study concerning operator support in steel production. The developed prototype is briefly presented along with justification examples.