The character of cosmological ionizing sources is still not 
established. At redshift beyond 3 star-forming galaxies seem to be 
the leading candidates in the production of ionizing radiation and the 
main responsible of the HI reionization of the universe (z>6). 

I will present recent results from two lines of research: (1) the 
investigation of the escape fraction of ionizing radiation of sources 
at moderate redshift (z~3-4), extracted from the GOODS-S/CANDELS 
survey, and (2) the deep VLT/FORS2 spectroscopic observations of Lya 
emission in galaxies at z>~7 and the implication for reionization. 
Finally I will discuss some prospects offered by the strong lensing to 
extend the above studies at fainter luminosities.