Our view of the structure of stars and gas in galaxies has greatly improved over the last few decades with the exquisite images provided by our modern space- and ground-based observatories. Surprisingly there are many basic details of our own Milky Way that are still unknown.
Although it is clear that we live in a late-type spiral galaxy, determining the exact number, size, and location of the arms is still a continuing effort today after years and years of detailed observations. Do we live in a "grand-design" galaxy with well-defined avenues of star formation along a few main spiral arms or is it "flocculent", with many arm-like structures that split and merge together? It is now fairly well-established that the Milky Way has a bar in its central few kiloparsecs, but what is its full extent and how does it interact with the arms and the Galactic Center? How flat is the Milky Way? Does it warp or flare at its edges?