In this paper we introduce results from two studies on frequency and knowledge of German idioms. The aim of the first study was to determine the frequency of idioms in a written language corpus, and in the second study the native speakers knowledge of idioms was exploited. In both studies selected idioms from dictionaries of idioms and dictionaries for German as a foreign language were used, and here we present the result of the comparison of the data from both studies concerning 1112 German idioms. One part of the result is a list with 143 highly frequent and well known idioms, that we propose to be a part of a "phraseological optimum" for German as a foreign language.