Abstract: Demand for food is quickly rising with increases in population and living standards. In the past few decades, crop yields increased rapidly due to the utilization of rice seedlings and mulching technologies, improvement of managements such as irrigation and fertilizer, new varieties selections and technology improvements. However, yields in farmer fields are much lower than potential yields, which have been widespread in the world's agricultural production. Therefore, closing the gap between current and potential yields is important to increase the crop yield and make sure the food security. In this study, the definition, research method, and the main results of yield gaps were reviewed. Furthermore, some prospects of yield gaps in the future were made, which will provide reference for further research on yield gaps. Until now there are many different definitions to yield gap, however, in general the maximum yield is the potential yield, and total yield gap is the difference between actual and potential yield. The yield gap caused by a variety of factors, including no-transferable technology and environment constraints, biological constraints (variety, diseases and insects, etc.), and socio-economic constraints (cost and returns, policy, knowledge, and tradition, etc.). In order to analyze the yield gap in detail, scholars divided the yield gap into different levels according to their objectives. There are two kinds of research methods for yield gap, the survey and statistical analysis methods, and crop simulation models method. The survey and statistical analysis methods have a simple concept and easy to be operated, but requires sufficient experiment data, which the cost is higher and the duration is longer; the crop simulation models method can design more scenarios using the computer, but can not quantify all of the management accurately. Therefore, in the yield gap researches, we should combine the statistical methods, crop simulation models and remote sensing technology should be combined for taking the advantages of each method. A comparison of the crop yield gap around the world indicates that for the developed countries, potential ascension of crop production is smaller due to the relatively higher levels of cultivation management. There are many studies on the yield gaps for crops around the world, which provide a scientific basis for enhancing the crop yield and closing the yield gap. However, there are large differences between their results because of different methods used. Due to the limitations of data and methods, most researches have been focused on the constraints of climate, soil, variety, and cultivation management factors on the yield in agricultural production, but ignored the wishes of farmers, policy and economic factors. Therefore, a subsequent study of crop yield gap should quantify the potential yield of the main crops in each region, and identify the constraints of climate, soil, variety, cultivation management, and socio-economic factors on the yield in agricultural production.

Key words: potential yield , yield gaps , yield constraints , research method