It was designed to highlight organisations where women were lagging behind and motivate change that would revolutionise workplaces.
But five years after the first set of gender pay figures were published by big employers, the latest numbers suggest a lack of progress. Women still earn on average about 90p at these organisations for every £1 a man does.
So is this any more than a box-ticking exercise?
Here are five things to take into account:
Measuring a company's gender pay gap means lining up the male employees in order of salary, doing the same for the women, and compare the two in the middle.
So the pay gap isn't just about whether men and women are getting equal pay for equal work (an area covered by law for 50 years).
It's also about how many women there are, what roles they hold , and if they're progressing up the ranks.
It may be about traditional career roles - hence why construction and finance, traditionally male dominated, have particularly large gaps.
Fields in which women are overrepresented may also have traditionally attracted lower pay. Store staff at Asda, for example, have been arguing in court that they should be valued the same as the largely male workforce in depots.
Gaps may reflect a lack of opportunities- for entering or progressing or corporate culture or attitudes. Or the challenges of responsibilities outside of work.
Some economists claim the gap is a "myth" , suggesting it may reflects women's choices, that they opt out of career progression or even the workplace.
But some may have the choice made for them: research commissioned by the Department for Business, Energy and Industrial Strategy found that 1 in 9 women said they had lost their jobs when they returned to work after having a child, or felt forced out of their job.
What is definitely true is that the gap opens up once children arrive - men and women earn roughly similar salaries in the early parts of their careers.
For employers, the challenge is hanging on to women, ensuring they have routes to progress.
For carers, disproportionally women, it's balancing responsibilities. And, ahead of last month's Budget which offered more financial help for childcare, financial services group PWC calculated that the challenge and costs of dealing with caring responsibilities in the face of a pandemic and soaring inflation may have halted women's workplace progress.
Later in their careers, a Chartered Institute of Personnel and Development report found that almost 900,000 women in the UK had left their jobs because of menopausal symptoms.
On the face of it, there has, after decades of progress, been no movement in the pay gap overall since big organisations started reporting these figures five years ago.
But some of those with the widest pay gaps admit the scrutiny has been constructive. Less than a decade ago, only 5% of EasyJet's intake of pilots were men - now it's 10%. The spotlight didn't only highlight the gap between pay in the cockpit and the crew in the back - but encouraged more women to become pilots.
Ironically, recruiting more women at entry level can cause the gap to widen before they move up the ranks.
Ask gender pay specialists and they'll tell you there are many initiatives that companies can take - tackling unconscious bias, offering more flexible working and encouraging shared parental leave.
But the issue doesn't end at the office door. The experts say society needs to change.
Schools could encourage girls to take more STEM subjects: science, technology, engineering and maths. The variety of flexible, affordable childcare options could be increased. And some men could take on more of the household chores.
But campaigners say gender pay gap reporting may not be enough. Some want legislations to force companies to explain how they intend to close gaps - with penalties for those who fail .
Over her career the average woman can expect to earn almost a quarter of a million pounds less than the average man. But that penalty aside, failing to capitalise on what women can offer may be restricting our wider fortunes at a time when skills are in short supply.
So is this any more than a box-ticking exercise?
Here are five things to take into account:
1: The gender pay gap is not the same as equal pay
Measuring a company's gender pay gap means lining up the male employees in order of salary, doing the same for the women, and compare the two in the middle.
So the pay gap isn't just about whether men and women are getting equal pay for equal work (an area covered by law for 50 years).
It's also about how many women there are, what roles they hold , and if they're progressing up the ranks.
2. There are many reasons the gender pay gap might exist
It may be about traditional career roles - hence why construction and finance, traditionally male dominated, have particularly large gaps.
Fields in which women are overrepresented may also have traditionally attracted lower pay. Store staff at Asda, for example, have been arguing in court that they should be valued the same as the largely male workforce in depots.
Gaps may reflect a lack of opportunities- for entering or progressing or corporate culture or attitudes. Or the challenges of responsibilities outside of work.
Some economists claim the gap is a "myth" , suggesting it may reflects women's choices, that they opt out of career progression or even the workplace.
But some may have the choice made for them: research commissioned by the Department for Business, Energy and Industrial Strategy found that 1 in 9 women said they had lost their jobs when they returned to work after having a child, or felt forced out of their job.
3: Getting more women in the door isn't the whole answer
What is definitely true is that the gap opens up once children arrive - men and women earn roughly similar salaries in the early parts of their careers.
For employers, the challenge is hanging on to women, ensuring they have routes to progress.
For carers, disproportionally women, it's balancing responsibilities. And, ahead of last month's Budget which offered more financial help for childcare, financial services group PWC calculated that the challenge and costs of dealing with caring responsibilities in the face of a pandemic and soaring inflation may have halted women's workplace progress.
Later in their careers, a Chartered Institute of Personnel and Development report found that almost 900,000 women in the UK had left their jobs because of menopausal symptoms.
4: Gender pay gap figures may obscure changes - but drive change.
On the face of it, there has, after decades of progress, been no movement in the pay gap overall since big organisations started reporting these figures five years ago.
But some of those with the widest pay gaps admit the scrutiny has been constructive. Less than a decade ago, only 5% of EasyJet's intake of pilots were men - now it's 10%. The spotlight didn't only highlight the gap between pay in the cockpit and the crew in the back - but encouraged more women to become pilots.
Ironically, recruiting more women at entry level can cause the gap to widen before they move up the ranks.
5: Don't just blame companies
Ask gender pay specialists and they'll tell you there are many initiatives that companies can take - tackling unconscious bias, offering more flexible working and encouraging shared parental leave.
But the issue doesn't end at the office door. The experts say society needs to change.
Schools could encourage girls to take more STEM subjects: science, technology, engineering and maths. The variety of flexible, affordable childcare options could be increased. And some men could take on more of the household chores.
But campaigners say gender pay gap reporting may not be enough. Some want legislations to force companies to explain how they intend to close gaps - with penalties for those who fail .
Why bother?
Over her career the average woman can expect to earn almost a quarter of a million pounds less than the average man. But that penalty aside, failing to capitalise on what women can offer may be restricting our wider fortunes at a time when skills are in short supply.