The systems biology approach, combining biological data with mathematical modelling, provides the means to elucidate the role of miRNAs in signalling pathways and gene regulatory networks. The approach is characterised by a cyclic process, including biological network construction, mathematical modelling, simulation and experimental validation. Using this approach, I first investigate the regulatory role of miR-34a in a signalling pathway. Second, the investigation of the regulation of a miRNA target hub by multiple and cooperative miRNAs is presented.