Abstract: Mobile agent technology is a new distributed computation paradigm. With its potential power, the technology aroused a great interest and experienced a rapid development in the last ten years. Various mobile agent systems were made for different targets and were implemented in different means. It is necessary to explore the current mobile agent systems and to summarize the implementation technologies for further research. In this paper a comprehensive explanation of mobile agent systems is given. First the definition and peculiarities of mobile agents and mobile agent systems are stated. Its advantages are enumerated. Then eight representative mobile agent systems are surveyed and the design issues about mobile agent programming language, mobility, communication, fault tolerance and security are discussed. Finally, the deficiencies and the trends of mobile agent technology are pointed out.