MicroRNAs (miRNAs) are an abundant class of small noncoding RNA molecules that inhibit the expression of cognate genes in multicellular organisms. These small RNAs have been demonstrated to play crucial roles in a variety of biological processes including cell differentiation, proliferation, and survival. Knowledge of specific expression patterns of miRNAs is critical for functional studies. Here, we describe a bioluminescence reporter gene-based method to measure miRNA activity in cultured cells and mice using a Gaussia luciferase reporter gene controlled by miRNA binding sites in its 3'untranslated region. This method can be used to noninvasively monitor the expression patterns of functionally active miRNAs involved in different biological processes or diseases in mice.