Discrete potential and function theories on a rectangular lattice and their applications The growing complexity of modern engineering problems necessitates development of advanced numerical methods. In particular methods working directly with discrete structures and thus representing exactly some important properties of the solution on a lattice and not just approximating the continuous properties become more and more popular nowadays. Among others discrete potential theory and discrete function theory provide a variety of methods which are discrete counterparts of the classical continuous methods for solving boundary value problems. A lot of results related to the discrete potential and function theories have been presented in recent years. However these results are related to the discrete theories constructed on square lattices and thus limiting their practical applicability and potentially leading to higher computational costs while discretising realistic domains. This thesis presents an extension of the discrete potential theory and discrete function theory to rectangular lattices. As usual in the discrete theories construction of discrete operators is strongly influenced by a definition of discrete geometric setting. For providing consistent constructions throughout the whole thesis a detailed discussion on the discrete geometric setting is presented in the beginning. After that the discrete fundamental solution of the discrete Laplace operator on a rectangular lattice which is the core of the discrete potential theory its numerical analysis and practical calculations are presented. By using the discrete fundamental solution of the discrete Laplace operator on a rectangular lattice the discrete potential theory is then constructed for interior and exterior settings. Several discrete interior and exterior boundary value problems are then solved. Moreover discrete transmission problems are introduced and several numerical examples of these problems are discussed. Finally a discrete fundamental solution of the discrete Cauchy Riemann operator on a rectangular lattice is constructed and basics of the discrete function theory on a rectangular lattice are provided. This work indicates that the discrete theories provide solution methods with very good numerical properties to tackle various boundary value problems as well as transmission problems coupling interior and exterior problems. The results presented in this thesis provide a basis for further development of discrete theories on irregular lattices.