Liquid-infused surfaces (or lubricant-infused surfaces) (LIS) are a new class of functional materials introduced in 2011. Their exceptional properties have earned them a place at the forefront of many fields including anti-biofouling, anti-icing, anti-corrosion, drag reduction, droplet manipulation and drop-wise condensation. Integral to their success is the infused lubricant layer which affords them their properties. In this review, we examine the current state of the literature relating to the lubricant layer. We consider the lubricant through all stages in the surface's lifecycle from design, to use, all the way through to depletion and eventual failure. First, we examine trends in lubricant choice and how to choose a lubricant, including environmental and medical considerations. We then look at the different methods used to infuse lubricant into surfaces and how lubricant depletes from the surface. We then report direct and indirect methods to characterise the thickness and distribution of the lubricant layer. Finally, we examine how droplets interact with LIS and the unique properties afforded by the lubricant before providing an outlook into where research centred on understanding the lubricant layer is heading in the new decade.