In the current competitive world@ Quality of Service (QoS) is becoming@ jointly with cost@ a key parameter in selling and buying telecommunications services. At the same time@ technology and liberalization trends are raising new types of concerns unknown with the Plain Old Telephony Services (POTS) using switched connections provided by a single monopoly supplier. Nowadays@ there are several standards describing QoS measurements but the question of which indicators are the most interesting to be monitored from the users point of view and which values they should meet is still open. The present document proposes a reference model to evaluate the Quality of Service from the users point of view@ defining the following concepts: a) the appropriate indicators for a QoS assessment from the user point of view; b) the methods to acquire the indicator values needed to assess the quality of service. The main principles for these definitions are: ? To define the services according to the applications performed by the user and not by the technical solution: for example@ voice over IP is one of the many technical solutions to communicate between subscribers of the world-wide telephone network; ATM@ frame Relay@ IP are some of the many technical solutions to ensure a data transmission service between a terminal and a server or between networks. The quality criteria are the same@ only the Service Level achieved can be different. ? To define the quality criteria with respect of usage and not technique. In speech quality@ users are more interested in intelligibility than in bandwidth@ distortion@ signal to noise ratio or lost packets. Quality criteria should be defined from the functional criteria@ then translated into technical criteria. This means that different performance parameters may be used to quantify and monitor the quality@ depending on those that are relevant for the technology used. Therefore@ the present document does not intend to describe measurement techniques since several ETSI TCs are dealing with such techniques and have the appropriate technical knowledge to develop standards in this area. EG 202 009-1 [23] gives guidance in identifying the indicators relevant from the user point of view. If these indicators and parameters are used in a Service Level Agreements (SLA)@ it is crucial to define@ at least for the most important ones@ the agreed quality targets. If they are used to compare the respective providers' performances@ then quality targets can be provided as guidance for the general public but what matters to the users are the results achieved. The present document intends to define user related service specific QoS parameters as far as possible using formal standards while EG 202 009-3 [24] proposes a template for a SLA dealing with all service aspects@ including penalties@ escalation procedures@ areas of responsibility@ etc where these indicators and parameters can be used. The purpose is to use the methodology described in EG 202 009-1 [23] to define@ for each QoS criterion@ the relevant indicators and parameters for a choice of services and for each step of the service life. Hence each customer can have a comprehensive information on the features of the service he intends to buy according to the various providers. This will enable him to select the best suited to his needs.