Issues concerning the use of object-oriented knowledgebases in hydraulic circuit design are discussed. Hydraulic system knowledge representation is described with a semantic association model. Hydraulic circuit design rules are categorized and modeled as integrity constraint or deductive rules. This work demonstrates that an object-oriented knowledgebase could potentially be a viable tool for providing the hydraulic circuit designer with an evaluation of the designed circuit. The techniques need further development to be useful by the practicing designer.