In the present study, the physicochemical and microbiological characteristics of the potable water in the prefecture of Kozani are studied, according to the results of the analysis carried out in the Environmental Centre Laboratory, during 2002-2005. According to the Greek Legislation and the EU Directive for potable water, a water quality monitoring program was created for the County of Kozani, and the parameters of the Check and Audit Monitoring were examined for a number of samples from selected samples sites at regular intervals. Conclusions relative to the water adequacy of springs and drills, water tanks and supply network, as well as the possible reasons for water quality problems are presented in the study. Generally, the water quality depends on its chemical and microbiological condition. In the areas with penetrable formations and agricultural activity, the nitrates presence was observed in relatively stable concentrations in time. The water is easily infected during its transportation from the source to the internal water supply network and finally to the consumer. The condition of the water supply networks is a factor which contributes to the water quality.