This chapter presents selected issues relevant from the perspective of psychological diagnosis conducted to assess the capacity to perform legal transactions. It presents general issues of diagnosis in the methodological context, indicating key areas, relevant from the perspective of conducting diagnostics related to the assessment of the capacity to perform acts in law. The chapter introduces selected tools from the field of diagnostics: neuropsychological, general cognitive performance and personality. There are no time restrictions in the test procedure and it is strictly standardised in terms of instructions provided to the participant and the diagnostician's behaviour. Diligence measures the degree to which a person is organised, persistent and motivated in intentional activities e.g. it describes attitude towards work. In such a situation, it is necessary to refer to more in-depth diagnostic process and formulate another hypothesis in an attempt to verify it in the subsequent stage of the diagnostic process.