MicroRNAs (miRNAs) are a novel class of small noncoding RNAs that regulate gene expression by targeting mRNAs for either cleavage or translational repression. They have been shown to play important roles in a broad range of biological processes including development, cellular differentiation, proliferation and apoptosis. Conventional detection methods, such as northern blot, real-time PCR or microarray, have been used to assess miRNA expression. However, these techniques require the fixation or lysis of cells, and thus cannot be used to study the dynamic function of miRNAs in living cells. Recent remarkable advances in molecular imaging techniques have provided the capability of noninvasive repeated quantitative imaging of tumour or stem cells in living animals. The current brief discussion focuses on the reporter and fluorescent beacon imaging approaches to visualize miRNA expression in living subjects.