We describe the objectives and organization of the CLEF 2006 ad hoc track and discuss the main characteristics of the tasks offered to test monolingual, bilingual, and multilingual textual document retrieval systems. The track was divided into two streams. The main stream offered mono- and bilingual tasks using the same collections as CLEF 2005: Bulgarian, English, French, Hungarian and Portuguese. The second stream, designed for more experienced participants, offered the so-called "robust task" which used test collections from previous years in six languages (Dutch, English, French, German, Italian and Spanish) with the objective of privileging experiments which achieve good stable performance over all queries rather than high average performance. The document collections used were taken from the CLEF multilingual comparable corpus of news documents. The performance achieved for each task is presented and a statistical analysis of results is given. Categories and Subject Des...