It is into such places that Christians in many countries go to minister to the prisoners, some of whom have waited years without their case being brought to trial.
From June 28 to July 2, Christians in prison ministry will gather from more than 100 countries for Prison Fellowship International's World Convocation in Toronto, Canada.
PFI President Ron Nikkel hopes it will be an encouraging time for the prison volunteers.
"The task of prison ministry is daunting," he said.
"World convocation provides a place where those of us working in prison ministries can come together and motivate each other.
"It is focused on how we can work more effectively, but also on the stories we have to tell."
There are around 50,000 PFI volunteers worldwide. In many of the 119 countries where PFI operates, prisons are beset with problems like extreme overcrowding, poor ventilation, food shortages, substandard sanitation and outbreaks of violence.
Christians often minister in these environments with little support, Nikkel said.
"Most people involved in prison work find themselves on the margins of ministry.
"They often lack the support systems – whether spiritual or material – that we take for granted in North America and Europe."
Speakers at this year's convocation include former president of the International Prison Chaplains' Association, Pierre Allard, author Philip Yancey, retired Anglican bishop and president of the National Unity and Reconciliation Commission in Rwanda, John Rucyahana, and human rights activist Theary C Seng.
The convocation will focus on restorative justice and look at the case of Rwanda, where victims and perpetrators of the Rwandan Genocide are learning to forgive and live side by side in "reconciliation villages".
The convocation will exhibit paintings, sketches and sculpture by prisoners created as part of an international competition.
Nikkel said: "There's a lot of hidden creativity in these prisons that doesn't get a chance to get out.
"The art competition encourages creative expression and gives a voice to the prisoners who can't come to a conference like this."