This dissertation shows the process of the disintegration of the Socialist Federal Republic of Yugoslavia itself, and describes all the causes and effects of that event. Through the cited literature all the key people and circumstances which have led to the disintegration of Yugoslavia are shown, starting from its founder and president Josip Broz Tito towards the final decision of the Arbitration Commission about the cessation of existence of Yugoslavia, i.e. the decision that ex-republics especially Slovenia and Croatia would have the right to independence and separation. In the second part of the dissertation, an overview of all the most important characteristics of the single-party system which had been based on the dictatorship of the Central Committee of the Communist Party of Yugoslavia, as its chief legal body, which had been retaining power through the control of all the spheres of social life is given. Afterwards, the characteristics of the multi-party elections which were introduced in Croatia after the disintegration of Yugoslavia, and the process and characteristics of the birth of new political parties and the new political system in Croatia are shown. In the last part the history of the activity of the HDZ from its foundations until the final victory at the first multiparty elections in Croatia which were held in April and May of 1990 are shown. A special emphasis is put on the programme of the HDZ and its contributions to the change of the political system in Croatia, and on the description of the most important biographical details of the first president and founder of the HDZ, dr. Franjo Tuđman who later on became also the first president of the sovereign and independent Republic of Croatia. At the end of the dissertation conclusions about the above-mentioned processes with a special overview of the cause and effect relation are listed.