Undermount sinks have a low profile and are attached underneath the countertop, hence their name. With no visible edges, undermount sinks offer the advantages of a streamlined look. The lack of a sink rim or lip over the countertop also makes cleaning easy, as food and dirt can't get trapped under the edges.
A wide range of sinks come in undermount styles: single-, double- and triple-bowl; stainless steel, composite, fireclay, cast iron, solid surface, stone and copper; and bar or prep sinks as well as primary kitchen sinks.
Undermount sinks expose the edge of the countertop to the bowl and the water within it, so make sure the countertop material you choose is waterproof throughout, not just on the top layer. That typically means using solid surfacing, concrete, quartz, granite, or other engineered or natural stones.
In addition to being more difficult to install than drop-in sinks, undermount sinks also are more expensive.