Hundreds of thousands are expected to gather in front of St Peters Bastillica in Rome, Italy tomorrow for the canonisation service of Mother Teresa of Calcutta,19 years after her death.

The ceremony will be the culmination of a process - sometimes called "the saint-making machine" - that is long, complex, expensive, opaque and often contentious.

The Missionaries of Charity, founded by Mother Teresa, gained her the Nobel Peace Prize.

It was here she selflessly cared for the dying, homeless and orphans gathered from the teeming streets of eastern India.

The Catholic Church posthumously confers sainthood on people considered so holy during their lives that they are now believed to be with God and can intercede with him to perform miracles.

Such is the status of the nun acclaimed for her work in the slums of the Indian city now known as Kolkata.

The Church spent many years investigating reported miracles attributed to the late Mother Teresa.

The first concerned an Indian woman, Monica Bersa, whose stomach tumor is said to have disappeared after she and others prayed to the nun in 1998, a year after Mother Teresa died.

The second miracle involved Brazilian Marcilio Andrino, who the Church says unexpectedly recovered from a severe brain infection in 2008 after his family prayed to Mother Teresa.

With two Church confirmed miracles in tow, the requirements for sainthood were met, but not everyone is convinced.

Critics, such as the late atheist writer Christopher Hitchens, who made a documentary called Hell's Angel on Mother Teresa, says the system is flawed.

One of the doctors who treated Bersa at the time, Ranjan Mustafi, told Indian media her healing was as a result of treatment.

Notwithstanding, the everyday work of the Missionaries of Charity goes on, and Mother Teresa's devotion to servicing the needs of the poor has without a doubt, impacted countless lives.