System identification is the theory and methods of establishing the mathematical models of systems.This paper discusses some basic issues involved in system identification,including the identification accuracy,the identification methods,the input signal design,the parameter identifiability and system identifiability,the open-loop identifiability and closed-loop identifiability,the identifiability and the controllability and observability,the identifiability and the input signal,and the excitation signal and the excitation conditions relate to the convergence of identification methods,and the convergene theorems of some typical identification algorithms.