One theme is individuality versus conformity. A lot of the people in Salem are reluctant to challenge the accusations of witchcraft because the religious fanatics and leaders of the town might accuse them as well. This is a clear case where people succumb to peer pressure, afraid to think reasonably and as individuals. That's why the word "hysteria" is often used to describe the infatuation with the witch allegations in this play. In this case, it means a psychological disorder of uncontrolled emotion, especially by a group of people. The town has become like one psychotic person. Since the majority of those in the town go along with the trials and executions, those individuals who actually speak out against the hysteria are either accused themselves or simply ignored.
It has been widely discussed that The Crucible is an analogy for the hysteria of McCarthyism which occurred during Miller's life. Senator McCarthy conducted a political witch hunt when he assumed the authority to collect and interrogate people he suspected of having communist ties or sympathies. This occurred during the early years of the Cold War, a time of growing mistrust between the United States and Russia, and a time when there was general fear of the possibility of nuclear war. There was good reason for this concern but McCarthy used this fear to justify his interrogations. Similarly, Danforth and others used fear of God and their own legal authority to execute in order to justify their witch hunt. So, another theme is the irresponsible use of power.