The influence of viscous damping and delay on the stability of haptic systems is studied in this paper. The stability boundaries have been found by means of different approaches. Although the shape of these stability boundaries is quite complex, a new linear condition, which summarizes the relation between virtual stiffness, viscous damping, and delay, is proposed under certain assumptions. These assumptions include a linear system, short delays, fast sampling frequency, and relatively low physical and virtual damping. The theoretical results presented in this paper are supported by simulations and experimental data using the DLR light-weight robot and the large haptic interface for aeronautic maintainability (LHIfAM).