Students across the UK face disruption as lecturers and support staff in 60 universities start an eight-day strike.
Members of the University and Colleges Union (UCU) are taking action in two separate disputes, one on pensions and one on pay and conditions.
The strikes will affect almost half of all UK universities.
The universities say strikes are not the way forward and promise to do all they can to minimise the impact of industrial action on students.
In addition to striking, union members are taking other forms of industrial action, including working strictly to contract, not covering for absent colleagues and refusing to reschedule lectures lost during the strikes.
This latest action follows strikes in February and March last year, meaning some students are being affected for the second time.
Staff taking action will walk out between 25 November and 4 December and the union has not ruled out further action next term.
UCU says staff have reached "breaking point" over a number of issues, including workloads, real-terms cuts in pay, a 15% gender pay gap and changes to pensions for staff in the Universities Superannuation Scheme (USS), which the union says will leave members paying in more and receiving less in retirement.
UCU general secretary Jo Grady said about 43,600 members would be taking strike action for "systemic change".
Dr Grady said the higher education sector had "made a lot of money over the past 10 years" but that spending on staff in that period had gone down and that there had been "an attack on working conditions in the sector".
The UCU is angry that members are now having to pay 9.6% in pension contributions, up from 8% and wants universities to pay the full increase instead.
The union estimates that, overall, changes to the pension could leave lecturers about £240,000 worse off in retirement, rising to £730,000 for professors.
But the University and Colleges Employers Association and Universities UK say employers have increased their pension contributions from 18% to 21.1% of salary, paying in an extra £250m each year.
The employers say even increasing their contributions to 22.7% of salary would cost them £373m a year.
They have warned that in order to meet the union's current demands, employers "would have to divert unsustainable amounts of money from other budgets with potential consequences including for jobs, student support, course closures and larger class sizes".
For the employers, Carol Costello of Liverpool University, acknowledged the union's concerns but said recent rises both in staff pay and in employers' contributions to staff pensions were at the limit of affordability.
"It's important that the... pension scheme trustees secure the benefits for the future of the 230,000 staff in the scheme," Ms Costello told the BBC.
"They've got to meet the legal requirements that the Pensions Regulator sets out.
"We believe that what the Pensions Regulator has said is that ultimately the level of contributions that we're putting in is at the limit."
At London's City University, the employers' arguments do not convince staff like Dr Claire Marris.
"I'm going on strike because I really care about the education that we deliver to our students and I feel our working conditions are being eroded," Dr Marris told BBC news.
She says that although she loves her teaching and research she feels her pay and pension "which is essentially deferred pay" are being whittled away.
"By giving us less money and expecting us to do more and more work, they're making it really hard for us to deliver the quality education and the quality research which we want to do and that we want to contribute to society.
"I want parents out there who are paying the fees for their children to know that 50% of the staff in universities today are casual staff, they are on short-term contracts, they are on hourly contracts and yet that's the money they are paying those fees for and I think that money should be invested in staff."
Lucy, a master's broadcast journalism student, is conflicted.
"I'm angry because I've paid all this money for this course... it makes it seem that this course is not important.
"Obviously I do understand why they striking... I do understand but there are different ways of going about it... I just don't think that's fair."
Grace, another trainee journalist, said losing a week out of such a short course was "really frustrating".
She believes strikes during her undergraduate course affected her final mark: "I had no dissertation tutor for six weeks... so for it to be happening again is just really annoying."
A young man in the first year of his undergraduate degree said while he supported the staff in terms of workers' rights "the timing isn't the best".
He has an assignment presentation due before the end of term but, with his seminar leader out on strike, he's not sure when or if it will take place.
"It is disturbing the rhythm of studying... it's a very pivotal time," he said.
The universities affected
The action involves members of the Universities Superannuation Scheme which covers staff in pre-1992 universities - those which had university status before former polytechnics became universities.
In total, the UCU says 43 universities are taking industrial action over both pensions and pay and conditions:
- Aston University
- Bangor University
- Cardiff University
- University of Durham
- Heriot-Watt University
- Loughborough University
- Newcastle University
- Open University
- University of Aberdeen
- University of Bath
- University of Dundee
- University of Leeds
- University of Manchester
- University of Sheffield
- University of Nottingham
- University of Stirling
- University College London
- University of Birmingham
- University of Bradford
- University of Bristol
- University of Cambridge
- University of Edinburgh
- University of Exeter
- University of Essex
- University of Glasgow
- University of Lancaster
- University of Leicester
- City University
- Goldsmiths College
- Queen Mary University of London
- Royal Holloway
- University of Reading
- University of Southampton
- University of St Andrews
- Courtauld Institute of Art
- University of Strathclyde
- University of Wales
- University of Warwick
- University of York
- University of Liverpool
- University of Sussex
- University of Ulster
- Queen's University Belfast
Staff at a further 14 institutions are striking over pay and conditions only:
- Bishop Grosseteste University
- Bournemouth University
- Edge Hill University
- Glasgow Caledonian University
- Glasgow School of Art
- Liverpool Hope University
- Liverpool Institute of Performing Arts
- Queen Margaret University, Edinburgh
- St Mary's University College, Belfast
- Roehampton University
- Sheffield Hallam University
- University of Brighton
- University of Kent
- University of Oxford
And staff at three universities are walking out in a dispute over pensions alone:
- Scottish Association of Marine Science
- University of East Anglia
- Institute of Development Studies