This report deals with cost estimation studies on electricity interruptions and voltage disturbances. It is divided in two parts; the recommended guidelines and the state of the art of methodologies. The guidelines give a short and practical description of how to execute a complete customer cost estimation study. They summarize the proposed approaches including specifications of customer groups, choice of cost estimation method and a description of the survey based and case based approaches. The guideline for cost estimation due to interruptions is mainly based on the survey based approach. Direct worth is recommended for Households, Industry, Commercial services and Public services. In addition, Contingent valuation is recommended for Households and Public services. An alternative cost estimation method for Households is Conjoint analysis. The interruption costs of Large customers and Infrastructure should be analyzed with a case based approach. The guideline for estimating costs of voltage disturbances focuses mainly on a case based approach. Reliable data can be collected with measurements and logging of events for Industry, Large customers and Infrastructure. Other customer groups have to be analyzed with the survey based approach. The report also gives examples of questionnaires and some considerations of country specific characteristics that need special attention at national level before implementing a nationwide cost study. The state of the art of methodologies gives an overview based on an extensive literature research regarding methodologies for revealing costs of quality of supply problems. Advantages and disadvantages of different methods, the design and conduction of questionnaires, and the analysis of the collected data are described. In addition, cost results of recent cost studies are presented.