This review is devoted to dynamical systems in fields of p-adic numbers: origin of p-adic dynamics in p-adic theoretical physics (string theory, quantum mechanics and field theory, spin glasses), continuous dynamical systems and discrete dynamical systems. The main attention is paid to discrete dynamical systems -iterations of maps in the field of p-adic numbers (or their algebraic extensions): ergodicity, behaviour of cycles, holomorphic dynamics. We also discuss applications of p-adic discrete dynamical systems to cognitive sciences and psychology.