The article discusses the relevant law in California, Illinois and New York, where State criminal courts have found a defendant's gang affiliation to be relevant to show motive; to show two or more persons were acting together and engaged in a common purpose and design; to show bias, interest, or other issues pertaining to credibility; and in several other circumstances other than to show propensity to commit crime. The article also discusses: (1) some of the evidentiary steps the prosecutor and gang expert witness must follow before a court will allow admissible gang testimony at trial; (2) some of the principal pitfalls of introducing gang evidence; and (3) strategy decisions that the prosecution and police should make prior to attempting to introduce gang evidence into a criminal trial. A court will generally admit gang evidence against a criminal defendant as long as the evidence is relevant to proving some fact other than the defendant's propensity to commit crime and the relevance of the gang evidence is not substantially outweighed by the danger of unfair prejudice to the defendant. Notes