Dependable information on large-area agricultural production and production estimation are essential for agricultural markets and the formulation of national and international agricultural policies. It can provide information and technical support for regional or global food security. Factors like worldwide climate change, increasing population and fast changes in land use/cover make the need more urgent. Traditional collection of crop information depends on huge in-situ investigation, which is expensive, time consuming and vulnerable to subjective difference. Along with the development in remote sensing technology and its application to crop information acquirement, some operational crop monitoring systems were developed and put into operation by several countries and international organizations. The development of major crop monitoring systems worldwide (United States, Europe, FAO, Canada, Brazil, Argentina, Russia and India) was reviewed and  introduced in detail. The paper points out that the crop acreage estimation, crop yield prediction, crop condition monitoring and drought monitoring are the four  primary themes in remote sensing based crop monitoring. In crop acreage monitoring, along with the development of remote sensing technology, the dependence of these systems on field survey has not been reduced, or even increased for some reasons. This is against the primary intention of remote sensing application: to reduce or substitute field survey. The potential of remote sensing in large-area crop monitoring has not been fully exerted. Independent crop yield predicting method with remote sensing is also in great need. How to increase the role of remote sensing will be the major direction for the development of remote sensing based crop monitoring systems.