In this paper work the main topic is Classification societies and their significance with reference to SOLAS Convention. The aim of this paper work is to emphasize the importance and significance of classification societies where are Det Norske Veritas (DNV) and Lloyd's Register of Shipping cited as two largest and Croatian Register of Shipping (HRB) as classification society in Croatia. Classification society takes care that vessels are in enough good condition to not endanger the safety of navigation. International Maritime Organization (IMO) and international conventions such as the International Convention for the Safety of Life at Sea (SOLAS), International Convention for the Prevention of Pollution from Vessels (MARPOL 73/78), International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals (IBC), and The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC) prescribes requirements and technical requirements regarding the airworthiness of a ship and other characteristics. Also, the aim of the paper is to explain the characteristics of heavy cargo ships (Ro-Ro, Lo-Lo, Flo-Flo). The conculsion of this paper work is to show that classification societies have big role and responsibility in the safety of navigation thanks to their technical knowlage, but the issue of navigation safety in recent decades has been significantly expanded and regulated internationally by relevant international conventions. Technology modernization has led to the development of three vessels forms used to transport heavy cargo named Ro-Ro, Lo-Lo and Flo-Flo ships that need to meet the regulations of classification societies and international conventions for the safe transport of people and cargo, and enviromental protection.