With the availability of new instrumentation for more complete ground motion observations, such as rotation from optical-based measurements, novel seismological applications arise. When combined with conventional inertial seismometer recordings, the new instruments allow one to observe six degree-of-freedom (6-DoF) ground motions, including three components of translational motions and three components of rotational motions. Such 6-DoF measurements are capable of delivering similar information compared to a small-scale array of triaxial seismometers. The applications of 6-DoF observations range from wavefield characterization, separation, reconstruction, tilt correction to seismic source bearing estimation, structural imaging and reduction of non-uniqueness in seismic inverse problems. In this talk, I will give an introduction to this 6-DoF seismology using illustrative synthetic and real examples and summarize some recent progress made in this relatively young field.