This book deals with the effect of public and semi-public companies on economy. In traditional economic models, several private companies - interested in maximizing their profit - interact (e.g., compete) with each other. Such models help to avoid wild oscillation in production and prices (typical for uncontrolled competition), and to come up with a stable equilibrium solution. The problems become very complex if we take into account the presence of public and semi-public companies - that are interested in public good as well as in the profit. The book contains theoretical results and numerical techniques for computing resulting equilibria. As a case study, it considers the problem of selecting optimal tolls for the public roads - tolls that best balance the public good and the need to recover the cost of building the roads. It is recommended to specialists in economics as well as to students interested in learning the corresponding economic models.