This chapter introduces the interactions between particles, a key input to the computer simulations described later in the book. Molecular level and fully atomistic interactions are described, having in mind particles forming liquid crystals phases. The empirical level models discussed comprise purely repulsive hard anisotropic particles (ellipsoids, spherocylinders) and attractive-repulsive (uniaxial and biaxial Gay–Berne type) ones. Expressions for electrostatic interactions and in particular charge, dipole and quadrupole ones are derived and typical values for some common mesogens provided. Dispersion interactions, molecular polarizability and chiral interactions are then introduced via quantum mechanical perturbation theory. Since liquid crystals are also formed by colloidal suspensions, dispersive interactions and Hamaker constants are briefly discussed, as well as model potentials for water useful for lyotropic systems, micelles and membranes.