The Duchess of Cambridge cut an elegant and stylish figure in a Reiss coat and Emilia Wickstead dress as she met workers at the charity's Centre for Addiction Treatment Studies in Warminster.
Today's visit was so Kate could see how the charity provides training, education and professional development for those working in addiction and related fields.
She will also attend a graduation ceremony.
Kate, 33, has been a patron to Action on Addiction since 2012. She is also patron to organisations including The Art Room and East Anglia Children's Hospices.
This is Kate's third public engagement this week: she attended the Diplomatic Reception at Buckingham Palace, wearing a tiara once owned by the late Princess Diana, and yesterday she joined Prince William at the annual ICAP Charity Day in London's City.
Yesterday, she showed off her newly cut hair after losing a few inches from her famous locks.
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