Light signalling mechanisms are crucial for plant adaptation to environmental changes: it has been found that both light and temperature cues are interpreted by closely interacting signalling pathways. In Arabidopsis thaliana, the seedling hypocotyl has emerged as an ideal model system to study light and temperature control of cell expansion. The light sensitivity is affected by the light intensity (fluence rate), an illustration of which is exemplified in the fluence rate response curve, and also the quality of that light (i.e. the ratio of red: far-red wavelengths of light). Here I have elucidated the phytochrome B (phyB) signalling mechanism in great detail and subsequently created a mathematical model of the red-light signalling pathway. This model is further extended to include changes within the ambient temperature range. Through application of stochastic modelling techniques it was possible to demonstrate the importance of dark reversion within the phyB mechanism in a diurnal cycle. Further, by considering alternative modelling methods and applying a highly integrated mathematical and experimental approach, it has been possible to show how the integration of light and temperature signals drives a molecular switch in transcriptional activity and hence control of cell expansion. This work highlights the importance of a multidisciplinary approach to molecular biology and describes the significance of signal convergence on molecular signalling.