|In this thesis, whether and how personality characteristics affect the performance in investigative interviewing and the efficacy of training was examined in a three-step research design. In Study 1, the structure of a 50-item aptitudes scale, a modified and extended version of the Police Interviewing Competences Inventory (PICI), was assessed using a general population sample (N = 300), and a four-dimensional aptitudes scale was created. The four dimensions found were named as Humane (13 items), Communicative-Insisting (13 items), Self-controlled (9 items), and Careful-Tenacious (10 items). In Study 2, student participants (N = 154) completed the aptitudes and the Five Factor Model (FFM) scales, and then interviewed witnesses who watched a mock robbery crime video. Interviewer performance was assessed based on the amount of details they could elicit, the perception of the witness, and researcher ratings of behaviours and question usage. Three dimensions of the FFM were correlated with the success measures: Agreeableness with witness perception and appropriate questioning, Extraversion with researcher ratings and inappropriate questioning, and Openness with researcher ratings. Only the Communicative-Insisting dimension of the aptitudes scale predicted high researcher ratings. In Study 3, we used a policing student sample (N = 38) to investigate the impact of training on the interview performance and also to analyze how training effect interacts with personality measures when predicting the performance of participants. Overall, training increased the performance of participants in most of the success measures. The Humane dimension of the aptitudes scale and the Openness/Intellect dimension of the FFM predicted training efficacy. The post-interview performance of the participants was predicted by the Openness/Intellect, Agreeableness, Extraversion, and Neuroticism (negatively) dimensions of the Big Five and the Careful-Tenacious and Communicative-Insisting dimensions of the aptitudes scale. Findings might help police departments to identify potential successful interviewers and develop new training policies.