Objective evaluation and the successful selection of texts should be an integral component of the theory of interpreter training. What makes a text easy or difficult? This is the problem the chapter at hand concerns itself with. We begin by exploring how interpreters (must learn to) understand. Next we discuss parameters of difficulty internal to the text – topic and structure, speech acts and levels of redundancy, cohesion, numbers and proper names, expressive effects such as quotes, Latin idioms, humour as parameters; visual aids, para- and nonverbal properties such as speaking velocity, preformulation and accent/ dialect as well as speaking style and errors. These parameters are explained individually and summarized in table form at the end of each section. The conclusion illustrates and summarizes the didactic relevance of discussing texts and their difficulty in interpreting.