M33 (The Triangulum Galaxy) is the third largest member of our local group. Only M31 and the Milky Way are larger. M33's broad spiral arms are particularly rich in star forming HII regions, perhaps more evident than any galaxy yet imaged. Its face-on orientation and proximity have allowed detailed study of the assortment of HII clouds populating its spiral arms. M33 has such an abundance of HII clouds, that many are separately cataloged as NGC objects. Images recorded in hydrogen alpha light have revealed numerous classic HII regions in addition to a general background of rings, loops, arcs, and filaments which permeates the structure of M33. Some of these less distinct structures are not associated with star clusters and are probably illuminated by a ubiquitous ultraviolet flux radiating throughout much of the spiral arm structure of M33. Some shell like structures represent enormous bubbles blown from the interstellar medium by powerful stellar winds and supernovae.

The brightest and most impressive of M33's HII regions is NGC 604. This massive HII region spans 1600 light years distinguishing it as one of the largest known HII regions and is often compared with another remarkable Giant HII region, 30 Doradus in the LMC. The area of NGC 604 could hold 40 Orion Nebulas! Close examination of the inner structure of NGC 604 reveals a large number of shells, filaments, and arches formed by the stellar winds of 200 O type giants plus several Wolf-Rayet stars. The main stellar population is only 3 million years old. NGC 604 is powered by an extended OB association lacking a central concentrated core like R136 in 30 Doradus. The massive stars of NGC 604 are scattered over a large area spanning the entire nebula and are 100 times less concentrated than R136. An endless number of tunnels and cavities allow the winds of massive giant stars to circulate through the caverns of NGC 604 causing huge bubbles that eventually blowout, forming filaments and arches and eventually new bubbles of gas. Because of its cavernous structure the fate of NGC 604 will not likely be the expansion and dispersion typical of other Giant HII regions. Its tenuous structure and abundant stellar population will allow it to sustain itself far longer then most HII regions.