Almost all metallic materials that have been put into practical use consist of polycrystals and contain grain boundaries. As a grain boundary possesses excess energy, so the area of the grain boundary tends to reduce, provided that the temperature is maintained at a suitable level for the atoms to migrate. This is the mechanism for crystal grain growth at high temperatures, and the process can be simulated by a computer on the basis of this principle. The photograph shows an evaluation of the changes in crystal grains which have occurred within a period of time for a two-dimensional triangular lattice with 200×200 lattice points. The condition at which each lattice point has a random orientation is regarded as the initial state, and the change in orientation at each lattice point when maintained at a given temperature is traced using the Monte Carlo method. The area consisting of lattice points having the same orientation represents the size of the crystal belonging to the same grain. The numerical figures represent the Monte Carlo step and correspond to the elapsed time, while different colors show different orientations. The photograph illustrates the process of a normal grain growing in an average manner over a period of time. Such calculations are made by changing the value of a factor which affects the initial state and the grain growth. By comparing the results of the calculations with those of experiments, it is possible to evaluate the most accurate values for each factor and to discuss the influence on grain growth. The grain size influences the principal properties of a steel material, such as its strength and toughness. Therefore, a major development area for the future will be to clarify the factors that govern grain size and to establish a method of controlling the grain size.