Prisoners, as all human beings, have all human rights. The only human right they are deprived of is freedom. Hence, the State is obliged to protect their human rights along with specific rights prisoners have when their situation is taken into consideration. One of their rights is to file a complaint if they feel deprived of their guaranteed rights. This thesis gives details about documents which regulate human rights and the rights of prisoners and it also describes complaint filing procedures in Croatia and the rest of the world. The purpose of this thesis is to acquire information from the Diagnostic Centre in Zagreb about the characteristics of prisoners who filed at least one complaint and to identify correlations of convicts and the characteristics of complaints. The research part of this thesis presents the following data about each prisoner: age and gender, reason why he/she is in the Diagnostic Centre, the criminal offence committed by he/she, criminal record and previous sentence serving etc. Afterwards, the data about the complaints themselves have been analysed. This includes: number of complaints, the body which received the complaint, subject of complaint etc. Finally, some of the correlations between the characteristics of prisoners and the subjects of complaints have been presented. Every piece of data comes along with comments, possible explanations and recommendations for future research. Considering the results, the author finds that there is plenty of room for progress. It is necessary to found a body, on Central Office level, which would keep conjoint statistical data about complaints and complainants. The data this thesis provides is a direct contribution to the Diagnostic Centre because it gives the Centre the opportunity to reduce the number of complaints thanks to the findings about the nature of complaints filed by a certain type of convict. Likewise, every criminal offence could be linked to its own analysis which could be a great contribution to future work.