M17 is one of the brightest and most massive star forming regions in the Galaxy. It is located at a distance of 1.98 kpc and associated with the highly obscured (A_V > 10) cluster NGC 6618 with more than 100 OB stars. The total mass of the complex (from CO observations) is 2×10⁴ M_⊙ and it can be split into M17 North (M17-N) and M17 Southwest (M17-SW). For M17-N the inferred density ranges between 10⁴-10⁵ cm^-3. The region seems to be externally ionized by the nearby cluster. M17-SW has been studied extensively in many different tracers and frequencies. It has an edge-on geometry and the gas is highly clumpy. The density at the cores reaches ~6×10⁵ cm^-3 and the UV field is ~10⁴ G₀. The high density clumps (< 10⁵ cm^-3) seem to be embedded in interclump material (~10³ cm^-3) surrounded by less dense gas (~300 cm^-3). A considerable fraction of the [C II ] material outside the center velocities is not associated with the gas traced by [CI] and CO. A recent SOFIA [¹³CII] study reveals unexpectedly large columns of warm and cold [CII].