Klopfzeichen is the debut full-length album by German experimental music trio Kluster.

Liner notes on the CD reissue on the Hypnotic label as well as some websites place the recording date precisely one year later in 1970.

In addition Kluster founder Conrad Schnitzler stated in interviews that the recordings took place during the same period as the first Tangerine Dream album, Electronic Meditation, which occurred in late 1969.

[3] The album was first reissued by Schwann in 1980 – with new cover art and a sticker touting Cluster and Conrad Schnitzler's previous membership in Tangerine Dream – and then on the U.S.-based Hypnotic label in 1996.

The album was again reissued on the Japanese Captain Trip label on April 20, 2007 as a 450 copy limited CD edition with an adaption of the original cover art-work and a bonus track by Eruption, "Black Spring", from their 1971 sessions.