Multiplex polymerase chain reaction (PCR) enables examination of many loci simultaneously, and this multi-locus genotyping information can increase interpretation capabilities beyond what is available from a single locus. Short tandem repeat (STR) profiles can be illustrated with simple triangles, line drawings, or more sophisticated tools for illustrating electropherograms. This chapter addresses single-source samples. An understanding of expected behavior of single-source DNA samples is important prior to attempting DNA mixtures containing multiple contributors