"Ach wie flüchtig, ach wie nichtig" (Ah how fleeting, ah how insubstantial) is a German Lutheran hymn with lyrics by Michael Franck, who published it with his own melody and a four-part setting in 1652.

Several Baroque composers used the hymn, including Johann Sebastian Bach, who wrote a chorale cantata.

It is part of the current Protestant hymnal Evangelisches Gesangbuch, and has also been used by 20th-century composers such as Ernst Pepping and Mauricio Kagel.

The lyrics of the hymn were written by Michael Franck after the Thirty Years' War.

His models were vanity poems by Andreas Gryphius, namely "Die Herrlichkeit der Erden / Muß Rauch und Aschen werden" ("The Splendour of the Earth / Will end in smoke and ashes").

[8] Johann Gottlieb Naumann used the final stanza for his Zeit und Ewigkeit (Time and Eternity).