The assessment of long-term ocean climate data records from satellites measuring in the thermal domain is a relatively new area of research. The situation is complicated by the fact that a set of all-encompassing requirements that addresses the needs of all potential users of the data does not exist. This chapter focuses on long-term sea surface temperature data records and considers the fundamental aspects of comparing satellite data with suitable reference data. Qualitative and quantitative methods that can be applied to provide objective assessments of individual long-term sea surface temperature data records are presented. These allow any user to evaluate which data record is the most appropriate for a specific application. We conclude with recommendations for future research.