This chapter is dedicated to the elementary problem, which concerns interactions between a single particle and the surrounding fluid. First, we explore the drag force, which is the most common interaction. It is shown how this force is derived and applied in practice. This topic is further expanded upon by introducing Basset and added mass force – both are crucial for unsteady cases such as accelerating particles. Next, lift forces (Magnus and Saffman) are shown that may result in the particle’s motion in the lateral direction. To some extent, this is associated with the next issue explained in the chapter: the torque acting on a particle. The following sections pay attention to other interactions: Brownian motion, rarefied gases and the thermophoretic force. These interactions play a role for tiny particles, perhaps of nano-size. Ultimately, we deliberate heat effects when the particle and fluid have different temperatures. Thus, this last section scrutinise convective and radiative heat transfer.