In this thesis the use of fuzzy logic techniques for condition assessment and diagnostic analysis of equipment in electric power systems is discussed. Condition assessment may be performed at a strategic level so as to assist in planning long and medium-term maintenance and replacement, or at operational level focusing on day-to-day maintenance. The first part of the thesis addresses the application of fuzzy systems for the purpose of strategic and operational condition assessment using the example of high-voltage circuit breakers. Due to the high dimensionality of operational condition assessment schemes, direct definition of the fuzzy rule base could become quite tedious; for this reason, an alternative approach is proposed here. The second part of the thesis discusses the application of fuzzy logic for diagnostic analysis in power systems, in particular for the purpose of Dissolved Gas Analysis (DGA). The standard DGA interpretation scheme, defined in IEC 60599, is extended by fuzzy reasoning, and the added value is demonstrated by a case study on high-voltage instrument transformers. Furthermore, for diagnostic analysis of power transformers, an adaptive fuzzy DGA interpretation scheme is developed on the basis of the standard scheme. The presented case studies aim at assisting decision makers in power system maintenance departments in developing fuzzy systems for condition assessment and diagnostic analysis.