Claudia-Liza Armah spoke to Wing Commander Hugh Milroy, who served 17 years in Royal Air Force, and served in the Gulf War.
He is now the Chief Executive of Veterans Aid, a position he has held since 2005.
Dr Milroy has been involved with the charity and homeless veterans since 1996, when he completed his MA. His Ph.D. was conducted among homeless veterans on the street.
In 2011, he was awarded an OBE for his contribution to the well-being of veterans.
He describes his experiences in the army as positive but recognises how hard it can be for many to make the transition, though the majority of veterans cope well.
He told The Headline Interview "in 2014, life on the streets is deadly" and explains the work the charity do to help those who have ended up there.
But notes that usually it isn't the transition from army life alone that causes the problems but other external factors exacerbating the problem.
Dr Milroy shares one particularly poignant encounter with an army veteran who had fallen out of society.
The memory of the encounter spurred him on to involve himself thoroughly in the charity.
He said: "That man haunted me. He haunted me."
Veterans Aid is one of two charities being supported by the Evening Standard,Independent and i Christmas appeal.
The charity last year provided more than 21,000 nights of accommodation for veterans in need.
The campaign will also be supporting ABF The Soldiers Charity.