Unmarried and same-sex couples can now adopt children together for the first time, as part of the biggest overhaul of adoption laws for 30 years.
The law applies to both heterosexual and same-sex couples
Previously, unmarried people in England and Wales adopted individually - giving their partner fewer parental rights.
New rights for foster parents and those tracing children they gave up are also part of the Adoption and Children Act.
Support groups say the changes, which were passed in 2002, could encourage more people to come forward to adopt.
Felicity Collier, chief executive of the British Association for Adoption and Fostering (BAAF), said the changes brought the law into the 21st century.
"It recognises the reality, which is that adoption is no longer about adopting babies relinquished by unmarried mothers, but much more about finding permanent families who are committed to children who are in public care."
She said many children had suffered difficulties, including neglect and abuse, in early life and the widest range of potential families was needed.
Some unmarried couples were previously put off adopting by the rules stopping both from having a legal responsibility, Ms Collier said.
"Every child has a right to a permanent legal relationship with both the people who are looking after them."
Under the act, foster parents can now apply for "special guardianship" orders, enabling them to take continuous responsibility for children until their 18th birthday.
And parents who gave up their children for adoption now have the right to try to trace them through an intermediary service - as long as the child wants to be put in touch.
National Adoption Agency chief executive Pam Hodgkins said adoptive children were often reluctant to trace their birth parents as they feared "being rejected a second time".
She said the change was "giving the impetus another way, it's giving another opportunity for people to say what they would like - and for the other person to respond".
Agencies will contact people who have been adopted to ask if they want to be put in touch with their birth parents, Ms Collier said.
No information on them would be given without their permission.
But she said the new law could help many people: "There's a generation of unmarried mothers who actually have lived wanting to know desperately whether the child they gave up for adoption is alive or dead."
Lorna Read, who recently traced her blood daughter, said it was right that children had the right to decide whether to be put in touch - and that advice was available to parents.
"Nobody knows who they're going to meet," she said. "They don't know who their child or who their parent is going to be and I mean they could be faced with a horrible shock."