Dynamical friction is a very interesting physical phenomenon, with important applications in astrophysics. At the simplest level, it can be described as the slowing down (“cooling”) of a test particle moving in a sea of field particles due to the cumulative effects of long-range interactions (no geometric collisions are considered). Several approaches have been devised to understand the underlying physics (which is intriguing, as the final result is an irreversible process produced by a time-reversible dynamics; e.g., see Bertin 2014; Binney and Tremaine 2008; Chandrasekhar 1960; Ogorodnikov 1965; Shu 1999; Spitzer 1987). In this chapter, the dynamical friction time is derived in the Chandrasekhar approach by using the impulsive approximation discussed in Chapter 7.