  Since the '80s the \textit{Einstein} observatory has shown the Young Stellar
Objects (YSOs), emit X-rays with luminosities, in the 0.3--8 keV bandpass, up
to $\rm 10^3$--$\rm 10^4$ times than the Sun and that the X-ray emission is
highly variable. ROSAT has confirmed the pervasiveness of X-ray emission from
YSOs and ASCA has provided evidence that the emission of Class I YSOs is
largely originating from optical thin-plasma at temperature of 1-50 $\times
10^6$ K. These intrinsic, unexpected, properties and the transformational
capabilities of the \textit{Chandra} and \textit{XMM-Newton} observatories has
made X-rays a powerful tool to trace the star formation process up to distance
of a few kpc around the Sun. Starting from the early evidences of the '80s and
the intriguing questions they raised, I will summarize the results obtained and
how they have influenced our current understanding of physical processes at
work and I will discuss some of the still open issues and some of the likely
avenues that next generation X-ray observatory will open.
