There are at least three quite different contributions to the friction force between two ideal surfaces, i.e., smooth surfaces sliding in the absence of wear: load-dependent friction (which depends on the surface structure or topography), adhesion-dependent friction (which depends on any adhesion between the two surfaces) and viscosity-dependent friction (which occurs when the shearing surfaces are separated by a thin layer of liquid). These contributions depend on the surface molecular structure, the contact area, applied load (or pressure), film thickness, film viscosity, and sliding speed (or shear rate). In addition to these purely surface or interfacial properties, the bulk properties of the materials such as the elastic modulus can also play an important role even in the case of wearless sliding of elastic surfaces. Thus, depending on their shape, the friction can be smooth or proceed via stick-slip. The situation with rough and/or viscoelastic surfaces can be very rich and complex, as can the adhesion between the surfaces, and both may depend critically on a combination of surface and bulk properties. The talk will review some recent experimental results, including theoretical modeling and computer simulations, on such systems, i.e., both rough and smooth, hard and soft, adhesive and non-adhesive, lubricated and unlubricated. Such studies are clarifying the molecular basis of many well-established tribological laws and empirical observations such as Amontons laws and the Stribeck Curve, and are also revealing new insights and relationships between tribological processes at the molecular and macroscopic levels.