I am in the material preview mode in Blender 2.8, using Eevee renderer. What material settings do I need to set, to get a transparent object in the viewport? The use-case is when modelling an object with reference side/front/top images - I'd like to be able to see the image behind the object that I'm modelling. Otherwise, the image is covered by the mesh that I'm working on.
Add a reference image
Enable transparency in Eevee
To make a material appear transparent in Material preview, use the Alpha value.
Don't forget to set Blend mode to Alpha-blend.
(If you cannot find these settings, switch to Eevee render engine in the render tab)