The subject of the study are markets with imperfect competition, which represent any deviation from the model of approximated perfect competition. The most important forms of such market structures are monopoly, oligopoly and monopolistic (limited) competition. Monopolies, except in rare cases, disturb the market equilibrium, which is why they are prohibited or strictly regulated by law in most countries. Oligopoly and monopolistic competition are the forms of market structure that we encounter most frequently in everyday life. Banks, pharmaceutical companies and certain sectors of the food industry are common examples of oligopolistic structures, while beauty salons, restaurants, publishing houses and supermarkets are examples of monopolistic competition. The main objective of the study is to theoretically define the structure of imperfect competition and explain its (negative) effects on the market as a whole. The example of Dubrovnik's public transportation company will be used to show when the existence of a monopoly is the best and most cost-effective solution. Although imperfect competition can be beneficial in rare cases, it generally leads to market inefficiency. This happens because the producer can cut production and raise prices, resulting in a price that exceeds marginal cost, underutilization of production capacity and production that falls short of the required economic optimum. The state regulates the behavior of companies through laws and regulations and thus ensures that the market functions smoothly.