Isinglass is a product of fish bladders and is mostly collagen. Some beers use isinglass for filtration to clarify beers by causing the yeast to fall out of suspension.

Originally it was from sturgeon, but is now extracted from cod fish. 

It's main use is in British cask conditioned ales, but other brewers may use it. Most commercial breweries use filters and homebrewers tend to use an algae called Irish Moss.

No one knows when isinglass started to be used, although there are several theories.

Isinglass is also used in China for fish maw soup. It is also said that the best source of isinglass originates in the South China Sea.