Facebook, Google, Apple, BT and Microsoft will all give evidence to the Independent Inquiry into Child Sexual Abuse over the next 10 days.
Alexis Jay, who chairs the inquiry, said it would consider "the adequacy of the response of the internet industry" to the online facilitation of abuse.
The hearing opened earlier on Monday.
Opening the proceedings, legal counsel Jacqueline Carey outlined some harrowing examples of child grooming online and its devastating impact on their lives.
She also told the inquiry:
