Drug laws which specify the amount of a drug in possession that determines the difference between a seller and a user require that laboratory procedures be applied in each case to determine the weight of a proscribed drug found on a suspect. Drinking-and-driving laws specify the amount of alcohol in the blood that constitutes legal intoxication, which presumably is related to individuals' physical capacity to drive under varying blood-alcohol levels. Research is required to determine the appropriate blood-alcohol level specified in such legislation. Applications of the scientific method to investigating crimes begins with induction followed by deduction, a cycle which is repeated until either the crime is solved or it is determined that the facts required to solve the case cannot be discovered. In the adjudicative process, clue materials analyzed by the physical sciences may provide evidence in three distinct ways: establishing an element of a crime, linking a defendant to the crime scene or victim, and contradicting or confirming testimony of a witness or a defendant. In linking physical evidence to victim-suspect contact, probability theory must be used to establish the probability of certain combinations of physical evidence occurring. The evidence in the Atlanta child murders is used to illustrate some of the principles discussed. Seven references are listed.