Chairman of the public inquiry into child abuse in Northern Ireland Anthony Hart has appealed for openness from residential homes alleged to be responsible.
Public hearings into claims of sexual, emotional, and physical abuse have opened in Banbridge, Co Down.
More than 300 witnesses are expected to be called over the next 18 months.
Mr Hart said victims would at last have the satisfaction of knowing that their stories were being listened to.
He said: "This may be a challenging process for everyone involved but it is our hope that everybody, whether from government or from the institutions, who is requested to assist the inquiry will co-operate in a fair, an open and whole-hearted way so that this unique opportunity will not be wasted."
The independent Historical Institutional Abuse Inquiry is hearing abuse claims in children's homes and juvenile justice centres over a period spanning more than seven decades.
It has the power to compel witnesses and refer evidence to the PSNI for criminal investigation.
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 was created in response to a campaign for justice by victims, which became increasingly urgent in 2009 following the findings of a similar investigation in the Republic of Ireland, which uncovered evidence of endemic abuse.
The executive government at Stormont has asked the three-member expert panel to examine allegations of physical, sexual and emotional abuse dating from 1922 to 1995.
A total of 434 people have contacted the inquiry to claim they were abused.
About a third of the applications are from people who are now living outside Northern Ireland, including Australia, Great Britain, the Republic of Ireland and other countries.
The public hearings in Banbridge are expected to last for 18 months.
Evidence is due from more than 300 witnesses, including former residents who claim they were abused as children, those who ran institutions, health and social care officials, and government representatives.
The inquiry's remit is limited to children's residential institutions in Northern Ireland, meaning alleged clerical sexual abuse outside the confines of homes, those in foster care or in laundries run by a religious order cannot be examined.
A lawyer for the inquiry is due to provide a general overview lasting until Wednesday, outlining the proceedings and the issues the panel is expected to address.
The first stage of public hearings later this month will concentrate on allegations made against two Catholic children's homes in Derry.
Nazareth House Children's Home in Bishop Street and St Joseph's Home in Termonbacca were run by the Sisters of Nazareth order of nuns.
Public hearings are due to finish in June 2015 with the inquiry team to report to the executive by the start of 2016.