Mission-critical settings impose stability and reliability constraints that are outside the norm for GIS data management and analysis tools. A case study on implementation of an outage management system at New York State Electric & Gas provides background for analysis of what it takes to build client/server GIS-based applications that absolutely, positively must be up and running 24 hours a day, seven days a week. Topics and issues for discussion include key lessons learned during the project; design, construction, organizational, and support considerations to meet 24x7 reliability needs; deployment and upgrade challenges; and architecture issues.