The principles of criminal procedure unquestionably have their widest application in the trial, which form the core of criminal proceedings. In order to bring to justice an offender who is to be fairly tried and, if found guilty, also fairly convicted, the fundamental principles in the pre-trial proceedings, i.e. the stage where the police authority plays its indisputable role, need to be respected. The police authority will achieve the desired objective (including individual and general prevention) only if it adequately reflects the statutory regulation of the principles which emerge from the pre-trial proceedings into the stage of trial. Knowledge of the principles in general, constitutional and international terms that influence (guide) the jurisprudence of the courts is a guarantee of the correctness of the police authorities' action. The contribution points out that the correctness of procedures should also be positively reflected in the decision-making practice of police authorities, where an undesirable deficit is currently found to be caused by underestimating the preparation of police officers within the crime detection and investigation section.