The Historical Institutional Abuse Inquiry (HIA) is examining allegations of child abuse in children's homes and other residential institutions in Northern Ireland from 1922 to 1995.
It is the biggest child abuse public inquiry ever held in the UK, having been contacted by more than 400 people who said they were abused in childhood.
The HIA inquiry was first announced in 2010 and was formally set up by Northern Ireland's first and deputy first ministers on 31 May 2012.
Its aim is to establish if there were "systemic failings by institutions or the state in their duties towards those children in their care".
It will also determine if victims should receive an apology and compensation.
The inquiry was established as a result of a campaign for justice, which gathered momentum in 2009 following the damning findings of a similar institutional abuse inquiry in the Republic of Ireland.
The Ryan Report, published on 20 May 2009, examined allegations of child abuse at Catholic-run children's institutions in the Republic of Ireland over a 60-year period.
It found that church leaders knew sexual abuse was "endemic" in boys' institutions, where children lived in a "climate of fear".
The Ryan report also concluded that physical and emotional abuse and neglect were features of Irish children's homes, reformatories and industrial schools and that the young residents faced a "daily terror of not knowing where the next beating was coming from".
The findings of the nine-year-long inquiry made headlines around the world and led to calls for a government-backed investigation north of the border.
Within months, thousands of people had signed a petition called Justice for the Victims of Institutional Abuse in Northern Ireland.
The petition was delivered to the Northern Ireland Assembly in November 2009, when MLAs backed a motion calling for a similar assessment of the scale of child abuse in Northern Ireland.
The Stormont Executive agreed to set up a public inquiry in 2011, and the following year, on 31 May 2012, the terms of reference were announced by Northern Ireland's first and deputy first ministers.
On the same date, retired High Court judge Sir Anthony Hart was appointed to chair the inquiry. MLAs subsequently passed legislation, giving the inquiry team statutory powers to compel witnesses to testify.
When it was originally announced, ministers initially said the HIA inquiry would examine abuse claims over a 50-year period from 1945 to 1995.
However, in October 2012, just as the first stage of the inquiry began, its remit was extended to investigate allegations dating back to the foundation of the state in 1922.
Since October 2012, the inquiry has been taking evidence in private session from former residents of children's homes, borstals, training schools and other childcare institutions.
The inquiry's remit is limited to children's residential institutions in Northern Ireland, regardless if they were run by the state, religious organisations or voluntary groups.
It means that victims of clerical child abuse that took place outside residential institutions, those abused in foster care and former residents of Magdalene laundry-style homes in Northern Ireland, are excluded from taking part.
A joint campaign by clerical abuse victims and ex-residents of laundry-style institutions to be included in the proceedings has so far failed.
In December 2013, a woman who claimed she was molested in foster care lost a legal challenge to force the inquiry to examine her case.
The HIA has three main elements that are known as an acknowledgement forum, a research and investigative team and an inquiry and investigation panel.
The purpose of the acknowledgement forum is to record the experiences of people who claim they were abused in children's institutions.
There has been some criticism of the lack of face-to-face counselling provided for those who have come forward to tell their stories.
Last August, the campaign group Survivors and Victims of Institutional Abuse (SAVIA) said some witnesses have been suicidal after giving testimony and claimed organisations like theirs were being left to "pick up the pieces".
But in a statement, a spokesperson for the inquiry said it had "dedicated witness support officers" and was making every effort to "ensure that sufficient emotional support is available for victims and survivors".
To date, 434 people have applied to testify to the inquiry or to speak to its acknowledgement forum.
More than a third of them are now living outside Northern Ireland, including 61 people who have moved to Australia.
The inquiry has established that between 1947 and 1956, about 120 children were sent from institutions in Northern Ireland to institutions in Australia, as part of a UK government policy of child migration.
A team from the HIA inquiry panel is making trips to Australia during the hearings in order to meet potential witnesses.
So far, the inquiry is investigating 13 residential institutions in Northern Ireland, but this number may be extended during the course of the public hearings.
The institutions under examination are a mixture of children's homes and training schools run by the state, by voluntary organisations and by the Catholic Church.
The inquiry does not have the legal power to find anyone guilty of a criminal offence, but if it receives information that a crime has been committed, the details will then be passed to the police.
The role of the the inquiry's research and investigative team is to compile and analyse all the witnesses statements gathered during the acknowledgement forum and submit a report - set within the historical context - to the inquiry and investigation panel.
The panel consists of the chairman and two other government-appointed inquiry members who will then produce a final report.
Their report will determine if there were systemic failures in the care of children and make recommendations on matters such as the possible need for a formal apology to victims and future compensation.
The HIA inquiry is due to complete its hearings by June 2015 and deliver its final report to the Northern Ireland Executive in January 2016.