The BeSSeL Survey is a Very Long Baseline Array (VLBA) key science project whose goal is to study the kinematics and spiral structure of the Milky Way Galaxy. They do this by measuring the trigonometric parallaxes and proper motions of masers associated with high-mass star forming regions throughout the Galaxy. The main maser emission lines include methanol (at 12.2 GHz) and water (at 22 GHz). Because the distance measurements produced by trigonometric parallax rely solely on geometry (and not assumptions about luminosity, extinction, etc), the BeSSeL survey has produced the most accurate measurements of R_0 (distance to the Galactic center) and Θ_0 (the circular rotation speed) to date. The link to the homepage of the BeSSeL survey can be found here.