This paper gives an overview of my PhD study into the persuasiveness of (legal) knowledge-based systems'. The results of three experiments show the possible problems that may arise when computerised legal decision aids are put into practice. The users in the experiments had great difficulties with judging the limitations of the legal knowledge-based systems they were using. For the average lawyer the competent use of computerised legal decision aids may prove to be more difficult than solving legal problems without the help of a computer. These results underline the importance of studying the usage of legal knowledge-based system.