We present a brief review of molecular biological basis and mathematical modelling of circadian rhythms in Drosophila. We discuss pertinent aspects of a new modelthat incorporates the transcriptional feedback loops revealed so far in the network of thecircadian clock (PER/TIM and VRI/PDP1 loops). Conventional Hill functions are not usedto describe the regulation of genes, instead the explicit reactions of binding and unbindingprocesses of transcription factors to promoters are probabilistically modelled. The modelis described by a set of ordinary differential equations, and the parameters are estimatedfrom the in vitro experimental data of the clocks' components. The model is robust over awide range of parameter variations. Through the sensitivity analysis of the model, roles ofVRI and PDP1 feedback loops are investigated, and it is proposed that they increase therobustness of the clock.