These mugs are very well-made double-walled borosilicate glass. They do a fantastic job in keeping beverages both piping hot and icy cold for much longer than it takes to drink them. I'm not sure why some people think they're flimsy. They are absolutely not at all! But they are glass and any kind of sharp edge in the sink or a drop from hand height directly onto tile or another hard surface will break them, though they likely won't shatter. Still, they are microwave and dishwasher proof, of a generous size and can really last forever if they're cared for properly.