The reionisation of the all-pervading intergalactic medium (IGM) is a landmark 
event in the cosmic history of structure formation. Still, despite much recent 
progress, a coherent description of the thermal state and ionisation degree of 
the IGM, the repository of most of the baryons across the history of the 
Universe, remains elusive. 
Most of our understanding of IGM physics, and its implication for galaxy 
formation and metal enrichment, depends critically on the properties of the 
cosmic ionising background. While it is generally thought that the gas is kept 
ionised by the integrated UV emission from active galactic nuclei  and 
star-forming galaxies, the relative contributions of these sources as a 
function of cosmic time are poorly known. 
While existing synthesis background models are successful in reproducing the 
main observational constraints, such as the HI ionisation rate vs redshift, the 
temperature evolution of the IGM, and the Thomson scattering opacity of the 
IGM, many uncertainties still exist, and background models are now facing hard 
challenges, when confronted to new observations both in the local Universe and 
at high redshifts.
In this talk I will review our current understanding of the cosmic ionising 
background, I will discuss the main uncertainties affecting existing models, 
and sketch possible strategies aimed at improving their reliability.