The most accurate instrument for spacecraft attitude determination is a star tracker. Generally, these are CCD- based instruments. Until recently, only first-generation units were available. However, these first-generation designs are limited to outputting positions of a few stars in sensor- referenced coordinates and require extensive external processing. Fortunately, advancing technology has enabled the development of a new second-generation class of star trackers. These designs are fully autonomous, solve the lost-in-space problem, have large internal star catalogs, use many stars for each data frame, have higher accuracy, smoother and more robust operation, potentially lower cost, and output attitudes which are referenced directly to inertial space without any further external data processing. Two currently available designs which are in production and meet these requirements are the AST-201 from Lockheed Martin Missile & Space and the ASC from the Technical University of Denmark. The first design is in the general size, power, mass, and reliability class of typical, conventional star trackers. The second one features reduced size, power, mass, and cost, with commercial off-the- shelf components. Second-generation star trackers have a promising future with a likely evolution to low cost, miniature, stock instruments with wide application to a growing variety of space missions.