He continually tortured his daughter, forced her into slavelike conditions and coerced her to perform sexual acts several times a day, often for hours at a time and sometimes he would turn off the electricity to punish her, leaving her alone in the darkness for days at a time.
She was completely dependent on him, and became a broken person by decades of abuse and rape. He also threatened her to kill her and the children imprisoned with her by gassing them if they tried to escape.
Elisabeth is not attending the trial and neither are her children or any other relatives.
By videotaping her witness statement and using it at the trial, the Austrian authorities are honoring a promise they gave her shortly after she was freed in April 2008.
The officer in charge had promised her she would never again have to see her father for as long as she lived if she would tell police the truth about her and her children's ordeal.
It was then that the horrific details of the crime came to light and Fritzl was arrested and locked up.
The defendant's lawyer, Rudolf Mayer, urged the jurors to see the human being in Josef Fritzl and not the monster, as the Austrian media has dubbed him.
"This is not a monster. He did not plan to murder the infant. I don't expect you to accept what he did, but please be fair and see him for what he is, a human being who has done wrong and who failed, but that does not make him a monster."
Over the years, Elisabeth gave birth to seven children. Three of her children were forced to live with her in her cell, never seeing daylight until they were freed last year.
The other "incest" children were allowed to live upstairs with Fritzl and his wife, Rosemarie, after he pretended "he found the babies on his doorstep" -- the "foundlings" were carrying handwritten messages from their "runaway" daughter asking her mother to take care of the kids, messages Fritzl had forced Elisabeth to write.
Despite decades of abuse and separation, Elisabeth and her children have been reunited and are trying to come to terms with their lives.
They have lived in the care of counselors and psychologists for the first eight months in an apartment inside a local hospital, protected against intruders and with a team of specialists looking after them.
The government has given them a new identity and they are living at an undisclosed location in Austria.
The trial, which has attracted 200 journalists from all over the world to the small town of St. Poelten, is scheduled to last for four days. A verdict is expected this Friday.
Incest carries a maximum sentence of one year in prison, but Fritzl faces life imprisonment if convicted of murder. If he is not convicted of murder, he could receive a maximum sentence of 15 years on the charges to which he pleaded guilty.