First stars (the so called Population III stars) form out of pure H/He gas, marking the crucial transition form a homogeneous, simple universe to a highly structured complex one. We have tried to constraint some of the proprieties of these objects on the basis of the (few) available observations: WMAP electron scattering optical depth, Cosmic Backgrounds, ultra deep field number counts. I will show that these constraints can be fulfilled if stars at any time form with a standard IMF in the range 1 Msun < M < 100 Msun.