The BeSSeL Survey (Bar and Spiral Structure Legacy Survey) is a VLBA Key Science project. The survey is named in honor of Friedrich Willhelm Bessel who measured the first stellar parallax in 1838. The goal of the survey is to study the spiral structure and kinematics of the Milky Way. We will accomplish this by determining distances, via trigonometric parallax, and proper motions of star forming regions in the Milky Way. The target sources are methanol and water masers that are associated with young massive stars and compact HII regions that trace spiral structure. With accurate distance measurements we will locate spiral arms, and with absolute proper motions we can determine the 3-dimensional motions of these massive young stars.

The goal of BeSSeL is to measure accurate distances and proper motions of up to 400 high mass star forming regions in the Milky Way between 2010 and 2015. This will result in a catalogue of accurate distances to most Galactic high mass star forming regions visible from the northern hemisphere and very accurate measurements of fundamental parameters such as the distance to the Galactic center (Ro), the rotation velocity of the Milky Way (Θo), and the rotation curve of the Milky Way.