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.

Invited chapter for the "Handbook of X-ray and Gamma-ray Astrophysics" (Eds. C. Bambi and A. Santangelo, Springer Nature, 2022), accepted (42 pages, 7 figures)