Spacecraft formation flying is an important application of networked dynamical systems. Attitude control of the spacecraft is an indispensable (many applications place strict requirements on it) and difficult (due to the nonlinear dynamics) problem being studied for decades. Attitude is defined as the rotation from one coordinate to a reference coordinate and Fig. 11.1 gives an illustration on the rotation from coordinate (xb, yb, zb) to coordinate (xa, ya, za). In interferometry applications, it is often essential to control different spacecraft to maintain the same or relative attitudes during and after formation maneuvers, thus bringing much attention to cooperative attitude control problems for multiple spacecraft.