This study is concerned with different forms of assessment of pupils' skills in experimentation. The findings of three studies are reported. Study 1 investigates whether it is possible to develop reliable multiple-choice tests for the skills of forming hypotheses, designing experiments and analysing experimental data. Study 2 compares scores from multiple-choice tests with scores from an open-response test in order to investigate how to assess pupils' skills in planning two-factor experiments. Study 3 investigates whether scores from multiple-choice tests correlate with student performance in a practical test on seed germination. (Contains 3 figures and 4 tables.)