The study notes that since many street drugs are not what users and sellers believe them to be, challenges to drug identity are fundamentally important to the defense in drug cases. In an analysis of these challenges this study integrates a discussion of the topics of instrument malfunction and interpretive standards as variables that influence the reliability of drug identification analyses. Thin-layer and gas chromatography are examined as illustrations of the problems of instrument malfunction. Results in such tests are noted to be subject to a great number of variables -- temperature, gas flow, dryness and thickness of the thin-layer plates, contaminants on the gas chromatography columns of pH, variability of the thin-layer tank, evenness of the layer of silica gel across the chromatographic plate, uniformity of the solvent in the chromatographic tank, etc. Similar problems are advised to arise if the analyst uses more sophisticated instrumentation, such as mass spectroscopy. The defense attorney is instructed to evaluate the specificity of the drug identification procedures and the specificity of the various tests. Drug identification procedures are divided into two main groups: fingerprint tests -- those which are specific for only on drug substance and therefore prove the prosecution's argument that the substance is what the defendant was charged with possessing; and screening tests -- those that, like hair color and eye color) are not specific. Examples of unreliable nonspecific tests are portrayed in the tests often used in marijuana and cocaine analysis. Appended are the names and addresses of crime labs participating in the LEAA Crime Proficiency Testing Research Program, which exposed the unreliability of the testing of a significant number of the labs. Ninety-two footnotes are listed. (Editor summary modified)