Perovskite oxides show many potential applications in the research fields of emerging materials and devices for electronics, information and communication because of their rich functionalities, e.g. magnetic, ferroelectric, multiferroic, mechanical and optical properties. Among them, ferroelectricity is currently being studied intensively due to the existence of many different mechanisms, and the coupling with magnetism and strain. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this review, we focus on the inorganic perovskite oxides to summarize the recent research progress of the improper ferroelectricity in general, but we review the magnitude of polarization, and the generation mechanism of improper ferroelectricity in perovskite superlattice, double perovskite structures and a specific SmFeO3 single crystal possessing antiferromagnetic domain walls in particular. This review will hopefully provide routes to systematically understanding the improper ferroelectricity.