The Duke and Duchess of Cambridge and Prince Harry are to take a ride on the London Eye as they mark World Mental Health Day.
William, Kate and Harry will spend the day talking to those who have battled with psychological problems - and hear about the help they have received.
At a reception in London, the royals will meet those whose lives were changed thanks to crucial support from a relative, colleague or friend.
Guests will be able to take selfies in a special photo booth in County Hall with a message thanking someone for being there for them, as part of a #Thereforme social media drive.
Kensington Palace said the Duke, Duchess and Harry were interested in learning about the kind of support that is most effective as they explore future projects through the Heads Together initiative.
Heads Together, which is being spearheaded by the royal trio, aims to change the national conversation on mental wellbeing and is a partnership between charities that provide frontline mental health support.
The Cambridges and Harry will take to the London Eye with some of the people who have shared their experiences with them.
This year's theme for World Mental Health Day focuses on the importance of psychological and mental health first aid for all.
Later in the day, the Eye will turn purple in support of World Mental Health Day.
The global Light Up Purple drive was set up in memory of Canadian teenager Amanda Todd, who killed herself in 2012.
The 15-year-old brought the problem of cyber bullying to mainstream attention after she posted a video on YouTube in which she told her story with handwritten signs.