The ability to withstand environmental temperature variation is essential for plant survival. Formative studies in Arabidopsis have revealed that light signalling pathways has a potentially unique role in shielding plant growth and development from seasonal and daily fluctuations in temperature. In this thesis we further investigate the integration of the light signalling networks and temperature signalling on the molecular level in Arabidopsis. First, we identified the transcript of the bHLH transcription factor LONG HYPOCOTYL IN FAR-RED 1 (HFR1) to be highly dependent on the ambient temperature and under strong control of the red light photoreceptor PHYTOCHROME B (phyB). We found that the long hypocotyl phenotype of the hfr1 mutant was exaggerated in warm conditions, specifically in blue light, downstream of cryptochrome 1. We further show that HFR1 acts in the warm by suppressing the function of PHYTOCHROME INTERACTING FACTOR 4 and 5 (PIF4, PIF5). PIF4 appears to act as a master regulator of several temperature responses and is directly regulated by the phytochromes. Thus, we define a molecular network where red light and blue light signals together with temperature merge on the regulation of PIF4. In the second part of this thesis, we investigate the relationship between temperature and the fluence rate of light on the inhibition of hypocotyl elongation in Arabidopsis. We find that the response to increasing fluence rates of light is highly dependent on the ambient temperature and that PIF4 and PIF5 acting downstream of the major red light photoreceptor, phyB, are essential for this response. In addition, we provide evidence that in cool conditions, PIF activity is under strong suppression by the gibberellin and HY5 pathways specifically at high fluence rates of red light. The collected work of this thesis highlights the importance of the PIF proteins as integrators of temperature and light signals and furthermore, demonstrates that the response to temperature is highly dependent on both the quality and quantity of light.