It is generally assumed that second language (L2) learners find it difficult to self-assess their pronunciation skills. In view of the benefits of self-assessment for the language learning process and the need to monitor one's pronunciation in independent learning environments, we investigated the reliability of self-assessments of pronunciation skills and set out to better understand the causes of difficulties. In our study, 46 advanced learners of German assessed their own articulation of different speech sounds in comparison with the sounds produced by a native speaker. In 85% of all cases the assessments of the raters and the self-assessments were identical. However, the learners only identified half of the number of speech sounds which the raters believed to be inaccurate. The study therefore confirms that even experienced L2 learners seem to find it difficult to self-assess correctly their pronunciation skills. In this paper we have explored a number of reasons for these difficulties in order to identify ways of further improving the self-assessment of L2 pronunciation. (Contains 1 figure and 1 table.)