In 2007 the idea was put forward that, through careful molecular design, it should be possible to synthesize liquids which contain permanent, well-defined molecule-sized cavities (pores). Such porous liquids could be a kind of liquid zeolite, or liquid MOF (metal-organic framework), exhibiting the size and shape-selective sorption (or dissolution) associated with microporous solids as well as the fluidity of liquids - a new and potentially useful combination of properties. However, these materials remained essentially hypothetical until recently. In 2014 and 2015 three papers were published which describe convincing examples of porous liquids, and studies have shown that they do exhibit some remarkable properties, such as very fast gas diffusion and high gas solubilities. The examples reported so far are almost certainly only the tip of the iceberg. Now that porous liquids are real', a new area of materials science may open up, with clear potential for long-term applications in chemical processes.