Czerny was brought as a boy by his father to meet Beethoven, and Beethoven was impressed by his prodigious talent.
He took him on as a pupil, and later he made Czerny teach his nephew Karl the piano.
Beethoven was deeply upset when Czerny reported that the boy had little musical talent.
Czerny was part of Beethoven's inner circle in Vienna, which included Ferdinand Ries and Stephan von Breuning, and was intimately acqainted with practically all Beethoven's piano works. He could play many of them from memory, and was a tireless advocate of them.
He is known to students of the piano today for his many hundreds of piano exercises, and in Volume 4 of his Complete Theoretical and Practical Piano Forte School he gave detailed instructions on how to perform each of Beethoven's major works.