A total of 188 samples, including 127 evidence samples, were analyzed by using either or both of the systems. The evidence samples used included fingernail scrapings, sperm or seminal fluid, epithelial cells, blood, and other tissues. This paper's description of the materials and methods encompasses the extraction and quantification of DNA, amplification and fragment analysis, and statistical analyses. Results are presented from seven cases in which the analysis of Y-STR's provided useful probative results that could not be achieved with analysis of autosomal STR's. The cases were as follows: two cases of forensic paternity, a mixture case, DNA from perspiration, fingernail scrapings, and two cases of semen positive but no sperm cells. Other sections of the paper discuss population studies and linkage with the autosomal loci. A reference database of Caucasian (n=517), African-American (n=535), and Hispanic (n=245) population groups within the United States was generated for estimating the haplotype frequency in forensic casework. Among the individuals profiled, 311 Caucasians, 412 African-Americans, and 194 Hispanics provided unique profiles in their respective population datasets. Linkage analysis found that the frequencies from forensically important autosomal loci can be multiplied with the Y-STR haplotype frequency. Y-STR loci can be useful in cases with multiple male assailants. In such cases, the number of male individuals can be more easily determined than with autosomal STR analysis. The results obtained with the Y-PLEXtm systems for several cases have been presented in U.S. courts for either "Daubert" admissibility hearings or at trial. In every case the results have been accepted by the courts. 11 tables, 2 figures, and 49 references