Blackleg is a seed-borne disease of potato caused by the soft rot bacterium Erwinia carotovora subsp. atroseptica in temperate regions. Although most seed stocks are extensively contaminated, blackleg incidence is related to seed contamination level, the threshold level for disease development being about 103 cells of E. c. atroseptica per tuber. Disease control relies primarily on the production and use of‘clean’seed potatoes. This is better achieved by planting seed potatoes with a low contamination level than by certification based on blackleg inspection and rogueing. Testing seed-potato stocks for E. c. atroseptica involves four steps, which have not all been fully evaluated. First, sampling to reflect variation in tuber contamination level. Second, tuber tissue is prepared for testing. Third, determination of numbers of E. c. atroseptica, which can be carried out using a selective diagnostic medium (CVP), immunofluorescence colony staining after immuno-capture of the target bacteria, enzyme-linked immunosorbent assay (ELISA) applied to tuber extract after enrichment and a quantitative polymerase chain reaction (PCR) assay. Although all methods have the necessary sensitivity level, they suffer from certain important drawbacks, including reliability, specificity and ease of use. Last, interpretation of contamination results in terms of blackleg risk assessment. These steps are discussed critically as a basis for future studies.