This thesis takes a deep look into the fundamentals of lubrication; the process in which the friction between two surfaces is reduced by adding a little bit of fluid in between, named the ‘lubricant’. The goal of this thesis is to improve our understanding of some relatively simple configurations, with the idea that it can serve as a proper foundation for more complex and practical situations. The most important parameter in a lubricated contact might be the thickness of the layer of lubricant. For lubrication layers we typically talk about micrometers or even nanometers. In this thesis, I will describe various different experimental setups that measure this layer thickness as a function of all sorts of experimental conditions. I differentiate between two types of lubrication: sliding and squeezing. The first part of this thesis will deal with sliding, meaning the two surfaces move parallel with respect to each other. The second part of this thesis will take a look at squeezing, when two surfaces move towards each other. Special attention is given to lubricants with complex properties, primarily shear thinning properties and viscoelasticity. In this thesis it is shown, both experimentally as theoretically, how shear thinning properties change the formation of the layer thickness and the generation of friction in sliding circumstances. Furthermore, it is found that viscoelasticity completely changes the way a liquid behaves when rapidly squeezed. The lubricating layer strongly resists being squeezed in this case, which gives an explanation for why these types of liquids are associated with noticeable ‘slipperiness’.