This paper reviews the results of the Chandra deep exposure observations on star forming regions .
The \rho Ophiuchi ( \rho Oph ) cloud cores B-F reveal \sim 100 X-ray sources above the detection limit of \sim 10 ^ { 28 } ergs s ^ { -1 } .
About 2/3 of the X-ray sources are identified with an optical and/or infrared ( IR ) object , including significant numbers of class I sources .
The class I sources exhibit higher temperature plasma than those of class II and III sources .
These features are confirmed with a larger number of X-ray sources in the Orion molecular clouds 2 and 3 ( OMC 2 and 3 ) .

Hard X-ray emissions are found from the sub mm - mm dust cores , MMS 2 and MMS 3 in the northern part of OMC 3 .
These cores show outflows in the radio and IR bands , hence are in a very early phase of star formation , possibly in the class 0 phase .
The spectra are heavily absorbed suggesting that the X-ray sources are embedded in the cores .

The giant molecular cloud Sagittarius B2 ( Sgr B2 ) exhibits more than dozen X-ray sources , two of which are associated with the HII complex Sgr B2 Main ( the ultra compact HII regions F-I ) .
These show an absorption of \gg 10 ^ { 23 } Hcm ^ { -2 } , which is the largest among the known stellar X-ray sources .
The X-ray source in the HII regions F-I shows strong K-shell transition lines from He-like and neutral irons , while that in the east of the HII region I has only a weak line .
No strong X-ray emission is found from any other HII complexes ; hints of weak X-rays are found from only Sgr B2 North ( the HII region K ) and South ( H ) .

The Monoceros R2 cloud exhibits X-ray emissions from young high-mass stars .
IRS 1 , possibly in zero-aged main-sequence , shows rapid time variability and a thin thermal spectrum of \sim 2 keV temperature .
Similar X-ray behaviors are found from younger high-mass stars , IRS 2 and IRS 3 .
These X-ray features are in contrast to the low temperature plasma ( \leq 1 keV ) and moderate variability found in high-mass main-sequence stars .

X-rays from 7 brown dwarfs and candidates are found in the \rho Oph cloud cores A–F , which comprise 40 % of those selected with the IR observations .
The X-rays are variable with occasional flares .
The X-ray spectra are fitted with a thin thermal plasma of 1-3 keV , with the luminosity ratio of X-ray to bolometric of 10 ^ { -3 } -10 ^ { -5 } .
These properties are essentially the same as those of low-mass pre-main sequence and dMe stars .