The care system helps older and disabled people with day-to-day tasks like washing, dressing, eating and medication.
It is under pressure across the UK after past governments failed to reform or fund the council-run system properly.
Experts believe the problems are now so acute that politicians who ignore the issue at this election, do so at their peril.
The system is devolved across the four nations, meaning parties need to develop solutions unique to their region.
1. Thousands fewer receiving care
Unlike the NHS, social care is means-tested. To receive help from your local authority, you must have a very high level of need and, in England and Northern Ireland, savings and assets of less than £23,250; in Wales, the threshold is £24,000. Scotland operates a different system.
Since 2010, the number of older people asking for council help has increased, but fewer now qualify for support in their own home or in care homes.
The charity Age UK estimates there are 1.5 million people in England who need some help with day-to-day life, but do not receive it.
2. Not just an elderly issue
Much of the debate centres on the increasing number of older people, but the number of working-age adults needing support is also rising.
A third of the people who are provided with long-term support in England are aged between 18 and 65, but the cost of their care is around half the total social care budget. This is because they often require more intensive - and therefore, expensive - support.
Councils can pay up to £2,000 a week or more to support someone who, for instance, has learning disabilities or a long-term physical condition. In many cases, the individuals will not have been able to build up savings or assets.
3. Funding is lower than 2010
Despite increasing demand, spending is still 5% lower than a decade ago because councils in England have had the money they receive from central government cut by nearly half.
Although councils have offset some of that loss by increasing local taxes, they still have nearly 30% less money to spend on all public services.
Local authorities have generally tried to protect their spending on adult social care, and the government has put in extra short-term money in recent years.
The Local Government Association says councils remain under intense financial pressure.
4. Fees for care differ across England
The fees that local authorities pay for care, both in a person's own home or care homes, vary hugely depending on where you live.
But across the country, the companies and charities who provide those services complain that councils are not covering the real cost of care.
5. People who don't qualify for free care pay more
The squeeze on council funding in England means that people who pay for themselves are often propping up the care system.
In 2016, the competition regulator estimated that in care homes, self-funders pay 41% more in fees than a local authority resident.
Increasingly, care companies warn they would go out of business without private clients subsidising the system.
Many of those self-funders will have ended up selling their family home to pay for their care. It is one of the reasons why many view the current system as unfair.
6. Scotland spends the most per person
The care systems across the UK are increasingly different in the way they operate, with England the least generous.
For instance, in Wales, no-one who is eligible for care at home is expected to pay more than £90 a week towards it.
In Northern Ireland, no-one over the age of 75 pays for home care.
Scotland provides free personal care for people who are assessed as needing support, whatever their age. In a care home, people receive £177 a week towards their fees, but if they have savings or assets above £28,000 they will have to fund the remaining costs.
These differences are reflected in the amount of money each nation spends per person on social care. There is no comparable data for Northern Ireland.
7. An ageing population causes problems
The UK has an ageing population and the pressures on the care system will only increase. While living longer is considered a positive, many older people will develop health conditions which mean they need help with day-to-day life.
For instance, the number of people with dementia in the UK is expected to more than double to 1.6 million by 2040.
The Alzheimer's Society has estimated that without local authority help, someone with dementia will typically have to spend £100,000 on the care they need.
For a long time they have called this a "dementia tax", as someone who develops the disease will have to cover their own costs until their assets are reduced to £23,250 - the threshold for qualifying for local authority help in England.
8. Lack of social care causes problems for the NHS
There is a knock-on effect for the health service if the care system is supporting fewer people, and only those with the highest needs.
Someone struggling to cope on their own is more likely to have a fall or neglect themselves, meaning they end up in hospital emergency departments.
Then, when they are ready to leave hospital, a lack of social care in the community can lead to them being marooned in a hospital bed for longer than required.
This is why experts say the future of the NHS is tied to creating an effective social care system.
9. Not enough care workers
Finding sufficient people willing to take on the challenging, yet rewarding, work of looking after people in their own home, or in care homes, is a continuing problem.
The latest official data for England shows there are 122,000 vacancies - just under 8% of the social care workforce in England.
Nearly a quarter of the 1.5 million people working in the sector are on zero-hours contracts (when employees are not guaranteed a full working week) and pay is often the minimum wage.
Many argue that improving the pay and skills of the workforce is vital to encourage people to consider care work as a career. Increasingly, social care workers are carrying out tasks that would once have been done by trained nurses.
10. Social care is creeping up the political agenda
Reforming the care system is a matter political parties have struggled to agree on.
Since 1997, there have been more than a dozen inquiries and government reports setting out a variety of ideas for funding social care in the longer term.
In England, none of those ideas have led to change. It is a complicated system that few understand, and in past elections, the issue has become a political football.
In the 2010 election, Labour's plans for reform were dubbed a "death tax". In the 2017 election, Conservative plans were labelled a "dementia tax".
An analysis of the number of times social care has been mentioned in Parliament shows just how discussion about what to do has increased over the years - highlighting the issue's urgency.
But, as Health Secretary Matt Hancock has said, with Brexit currently dominating the agenda, plans for reforming care have struggled to get the "bandwidth" in government.
Even so, the debate seems to have shifted away from earlier ideas of insurance schemes and caps on costs. In the summer, a Lords committee called for free personal care for those aged 65 and over in England.
It would be a similar system to the one introduced in Scotland nearly 20 years ago, an idea dismissed at the time as too expensive for England.