It is generally accepted that dynamic voltage scaling (DVS) is one of the most effective techniques of energy minimization for real-time applications. The effectiveness comes from the fact that the amount of energy consumption is quadractically proportional to the voltage applied to the processor. The penalty is the execution delay, which is linearly and inversely proportional to the voltage. According to the granularity of units to which voltage scaling is applied, the DVS problem is divided into two subproblems: inter-task DVS problem, in which the determination of the voltage is carried out on a task-by-task basis and the voltage assigned to the task is unchanged during the whole execution of the task, and intra-task DVS problem, in which the operating voltage of a task is dynamically adjusted according to the execution behavior to reflect the changes of the required number of cycles to finish the task before the deadline. Frequent voltage transitions may cause an adverse effect on energy minimizatuion due to the increase of the overhead of transition time and energy. In this paper, we survey and describe, in a theoretical aspect, state-ofart techniques of dynamic voltage scaling problems, which include: (1) inter-task DVS problem, (2) intra-task DVS problem, (3) integrated inter-task and intra-task DVS problem, and (4) transition-aware DVS problem.