A systematic approach to qualitative analysis at the down of the twenty-first century is presented. After a description of the most salient features of the binary yes/no response (types, quantitative connotations, properties and errors), the main types of qualitative analysis and the role played by standards and calibration are discussed. An overview of qualitative procedures based on classic and instrumental tools is provided. The significance of sample and analyte screening systems used to obtain binary responses is also discussed. Finally, the main present challenges of qualitative analysis are outlined.