Detecting interactions and resolving conflicts is one of the key issues for generative planning systems. Hierarchical Task Network (HTN) planning systems use critics for this purpose. Critics have provided extra efficiency and flexibility to HTN planning systems, but their procedural - and sometimes domain-specific - nature has not been amenable to analytical studies. As a result, little work is available on the correctness or efficiency of critics. This paper describes a principled approach to handling conflicts, as implemented in UMCP, an HTN planning system. Critics in UMCP have desirable properties such as systematicity, and the preservation of soundness and completeness.