The drugs most often involved in DUID cases are discussed; the effects of the major types of drugs (depressants, stimulants, and psychedelics) on driving behavior and in arrest situations are described. Slang terms for a variety of street drugs are defined, and synergistic and antagonistic effects of various drug combinations are described. Investigative guidelines in such cases then are provided. It is suggested that police should seek to obtain a confession or admission from the defendant, check for needle marks as evidence of drug abuse, search the defendant's car and person, and conduct a record search. In addition, the officer should determine the defendant's prior medical history, have a trained narcotics officer view the defendant, arrange a medical examination, and document the physician's and laboratory findings. Specific issues relevant to each of these investigatory procedures are examined in the pertinent case law. Finally, pretrial and trial issues are discussed with reference to both statutory law and judicial decisions, and evidentiary requirements for conviction are delineated. A sample police report and subpoena for a DUID case are appended. 123 footnotes.