This monograph is devoted to the solution of various problems in the theory of differential equations in the space M'' of bounded numerical sequences (called countable systems). In particular, the general theory of countable systems, the theory of oscillating solutions, and the theory of countable systems with pulse action are treated. Main attention is given to generalization of the results of numerous authors, obtained in recent years for finite-dimensional systems of different equations to the case of systems from the analysed class.