A judge today accepted James' Holmes plea of not guilty by reason of insanity.
Holmes is accused of the mass shooting in a Colorado movie theater that killed 12 people and injured 70 others.
He has been charged with multiple counts of murder and attempted murder.
Prosecutors are seeking the death penalty.
It is estimated that Holmes' mental evaluation could take months.
Prosecutors say Holmes spent months buying weapons, ammunition, and explosives, and scouted the theater in advance.
If jurors find Holmes not guilty by reason of insanity, he would be committed indefinitely to the state mental hospital. He could eventually be released if doctors find his sanity has been restored, but that is considered unlikely.
If jurors convict him, the next step is the penalty phase, during which both sides call witnesses to testify about factors that could affect why Holmes should or shouldn't be executed.
A four-week trial is set to begin Aug. 5