MicroRNAs (miRNAs) are small, noncoding RNAs that regulate protein levels post-transcriptionally. miRNAs play important regulatory roles in many cellular processes, including differentiation, neoplastic transformation, and cell replication and regeneration. Aberrant expression of miRNAs has been linked to infectious and regenerative disease and carcinogenesis, among others. Recent studies as well have reported that significantly expressed miRNAs in serum are related to cancer and a range of infectious disease. With this in mind, several investigators have made attempts to apply circulating miRNAs as useful clinical biomarkers. In this chapter, we explore making diagnoses using miRNA expression pattern with the hope that this method will lead to higher levels of accuracy.