The Triangulum Spiral Galaxy Messier 33 offers unique insights into the building of a galactic disk .
We identify spectacular arcs of intermediate age ( 0.6 Gyr - 2 Gyr ) stars in the low-metallicity outer disk .
The northern arc spans \sim 120 degrees in azimuth and up to 5 arcmin in width .
The arcs are located 2-3 disk scale lengths from the galaxy centre ( where 1 disk scale length is equivalent to 0.1 degrees in the V-band ) and lie precisely where there is a warp in the HI profile of M33 .
Warps and infall are inextricably linked ( Binney , 1992 ) .
We present spectroscopy of candidate stars in the outer northern arc , secured using the Keck I telescope in Hawaii .
The target stars have estimated visual magnitudes as faint as V \sim 25m .
Absorption bands of CN are seen in all spectra reported in this review talk , confirming their carbon star status .
Also presented are PAH emissivity radial profiles generated from IRAC observations of M33 using the Spitzer Space Telescope .
A dramatic change of phase in the m =2 Fourier component is detected at the domain of the arcs .
M33 serves as an excellent example how the disks of spiral galaxies in our Universe are built : as dynamically open systems , growing from the inward , outward .