Power remains one of the crucial themes of Macbeth. The theme of power and ambition to gain it is evident in almost all the characters in the play - but primarily Macbeth himself.
At the beginning of the play, Macbeth has enough power - he is a good, brave soldier. Upon hearing the witches' prophecy, however, he realizes that it is not enough for him to be a soldier. By blindly following the prophecy and his wife's instructions, he becomes less powerful. Both the Three witches, and Macbeth's wife, therefore, have control over his life at this point. It is Lady Macbeth that convinces Macbeth to kill Duncan in order to become a king, even though Macbeth is clearly having second thoughts about it. At this point of the play, Macbeth's wife is more decisive and powerful than him.
When Macbeth kills Duncan and becomes the new king, he does develop power, but not for long. He begins to feel guilty because of the murder he committed. This reduces his power - he becomes weak, he hallucinates, and is unable to think rationally, which leads to his death. Macbeth's character, therefore, undergoes the phases of having enough power as a soldier, having less power when listening to his wife, having more power as a king, and eventually losing all his power and dying.
It might be useful to note that other characters (Macduff and Malcolm, for instance) might not seem that powerful at the beginning of the play, but gradually gain more power. They use their power only when they have to - Macduff uses it to revenge his wife and children, murdered by Macbeth. He is wiser and more modest. At the end of the play, Macbeth dies, and Macduff and Malcolm seize power.
