The hemispheric dichotomy is one the most fundamental and least understood features of Mars. The papers in this special section address aspects of the origin of the crustal dichotomy and the modification and evolution of the dichotomy boundary and the northern lowlands. These studies utilize new data from a variety of instruments on spacecraft operating in Mars orbit and on the surface. This special section is an outgrowth of a two-day workshop held at the Lunar and Planetary Institute in Houston, Texas.