It may come as little surprise that the latest prison disturbance erupted at HMP Birmingham.
It's in the top 20 of the UK's most overcrowded jails and its 1,469 inmates form the fourth biggest population of all prisons.
Other Government statistics show there were 518 assaults there last year, the fifth highest and 61 of those were classified as serious.
Although a snap official inspection two years ago found that its transfer to private control under G4S three years earlier had gone well, there was still a high level of illicit drugs and a risk to new inmates because of strained induction procedures.
Last year, outgoing chief prisons inspector Nick Hardwick said jails in England and Wales were in their worst state for 10 years.
The Prison Officers Association (POA) has long warned that the prison service is in crisis, caused by understaffing that has prompted a rise in violence and suicide.
Dwindling staff and rising prisoner numbers mean inmates' time out of their cells is often reduced, which builds up resentment and isolation.
Last month the Ministry of Justice revealed that six inmates a week die in British prisons, a 38% increase in two years.
Justice Secretary Liz Truss responded with plans to recruit an extra 2,500 staff and promised a crackdown on the use of drugs and mobile phones inside prisons.
She also said prison officers would be kitted out with body-worn cameras to increase their confidence and deter attacks.
A series of riots has illustrated the POA's concern: the last was six weeks ago at Bedford prison where 200 inmates took over parts of the jail for several hours, even posting mobile phone footage online.
At north London's Pentonville prison an inmate was stabbed to death and two others later escaped.