Current digital cellular and cordless telephone systems are called the second generation systems to the analogue mobile communication systems. According to the statistics on the current cellular systems and its users in the world, the total number reaches more than 450 millions at the beginning of this year. Since the population on the earth is 6 billions, the penetration rate is 8% today. The statistics also shows that roughly 70% users are now use digital systems, and the number of users increases very dynamically. On the other hand, analog systems are now finishing their work. To realize multimedia mobile services and worldwide roaming capability, the third generation system, IMT-2000, is being developed in conjunction with the ITU standardization activities. The service is planed to start around the year 2001 subject to market considerations. IMT-2000 is expected to play the key role of the mobile telecommunication infrastructure for providing multimedia services supported by user bit rates up to 2 Mbit/s. Services that are provided by different second generation systems will be integrated in IMT-2000. Following the expansion of Internet services, the needs for data transmission in fixed services is increasing very rapidly. Therefore, more kinds of application services and more high-speed transmission capability, e. g. more than 2 Mbit/s, are expected for future mobile communication systems. Considering this perspective, the studies of the fourth generation(4G)system have been started in various organizations. This paper gives an overview of the deployment history of mobile communication systems and discusses the studies required for developing the fourth generation system, especially from the viewpoint of frequency spectrum related issues. To realize high-speed more than 2 Mbit/s services into commercial use, the microwave and millimeter wave frequency bands will be required in order to further widen the bandwidth. Breakthrough technologies will be needed to overcome the increase in path loss and the delay spread seen in the high mobility environment. Antenna beam narrowing is one of the effective techniques that can cope with these problems. Narrowing the beam width increases the antenna gain to compensate the increased path loss, and radiation only in the desired direction reduces the delay spread. To adopt narrow beam antennas, the base and mobile station needs the beam tracking function to adjust beam direction. As the base station must communicate with multiple mobile stations in different directions, the base station antenna must be a multi-beam antenna. Using an adaptive array antenna in combination with an equalizer may realize powerful transmission quality improvements in the time and space domain. The adaptive pre-coding for TDD operation, adaptive time slot allocation between frequency channels and beams, and software radio for hybrid access techniques will be also effective for signal transmission in the fourth generation system. The information on the direction of arrival and time of arrival in microwave and millimeter wave frequency bands will become key factors in the enhancement of these techniques. Considering the deployment history of mobile communication systems, we should immediately start studies on the overall objectives, service applications, and technical, operational and spectrum related requirements for 4G system. Taking into account the huge amount of required frequency spectrum for 4G system, we have also to start spectrum studies to find the compromise solution for future radio services and contribute the results to the world radio conference(WRC). Like the decision on the frequency bands allocation for IMT-2000 at the WARC'92, the indication of appropriate frequency bands and bandwidths for 4G system by WRC will make progress in the R&D studies and to ensure the smooth deployment of 4G system.