This paper presents DEPTH (usability evaluation based on DEsign PaTterns & Heuristics criteria) a methodology for performing scenario-based heuristic usability evaluation of Web sites. It is comprised of two operational phases, the preparatory and the execution phase, where specific steps are performed for evaluating the usability along three axis: usability comparison tests among similar Web sites, expert/heuristic evaluation, scenario-based user inspection/enquiry sessions. DEPTH’s innovation lies on the use of usability design patterns both as a mean for identifying best practices of usability design as well as for specifying the scenarios and tasks for user inspection. It is also a method for capturing, adapting and refining usability resources of a Web site. DEPTH is also supported by a CASE tool whose functionality is described. Finally, we present a small example demonstrating the basic steps of DEPTH.