Irving John (Jack) Good (born 9 December 1916) is a British statistician who worked also as a cryptographer and developer of the Colossus computer at Bletchley Park. In his publications he is called I. J. Good.

He was born Isidore Jacob Gudak, in a Jewish family in London. He read mathematics at Jesus College, University of Cambridge, graduating in 1938. He did research work under G. H. Hardy and Besicovitch, before moving to Bletchley Park in 1941 on completing his doctorate.

At Bletchley Park he was initially in Hut 8 under Alan Turing; he worked with Donald Michie in Max Newman’s group on the Fish ciphers, leading to the development of the Colossus machine.

After the war ended he worked at Manchester University and then at GCHQ until 1959. He then had a variety of defence, consulting and academic positions. He was a prolific author of technical papers.

In 1967 he moved to the United States. As of 2004, he is a University Distinguished Professor at Virginia Tech, where his "vanity" car license plate, hinting at his spy-like wartime work, is "007 IJG".

He is known for his work on Bayesian statistics. He has published a number of books on probability theory.

He played chess to county standard, and helped to popularise Go, a Asian boardgame, by a 1965 article in New Scientist (he had learned the rules from Turing).