Several methods suggested for improving juror comprehension and application of the judge's instructions on the law are the use of written instructions to accompany the jury into deliberation, an oral precharge at the start of the trial to introduce jurors to their task and to the focal legal and evidentiary issues under consideration, the use of special questions or interrogatories to accompany the jury into deliberation, and the use of written notes taken by the jurors during the trial as an aid in deliberation. The effectiveness of these different methods was tested using 53 six-person mock juries created by sampling citizens serving in the Massachusetts Superior Court jury pool in Middlesex County during January through August 1979. Each of the mock juries was shown a film of a reenactment of an actual trial and the asked to deliberate to a verdict. The manner in which instructions were given to the mock juries was varied at random, so that each group had an equal chance of receiving any one of the methods. Questionnaires issued to jurors showed that the most useful decision aid was the written copy of the trial judge's instructions provided during deliberation. The use of handwritten notes taken by jurors during the trial should be discouraged. A total of 34 references and tabular data are provided.