This is one of the biggest moments in Britain's economic calendar - bigger, in some respects, than the annual budget.
Yet trying to get your head around the spending review - in particular this year's spending review - is a far more fraught exercise than with the budget.
So perhaps the best place to start when approaching the review is to take a deep breath and a step back. With that in mind, here are five things you really need to know about the 2025 spending review.
1. It's not about all spending
Around 59p in every pound of government spending (which amounts to 拢1.4trb this fiscal year) goes on things like welfare, pensions and debt interest - meaning it is largely out of the government's control.
This kind of spending isn't being discussed by the government today - only departmental spending.
2. It's a 'zero-based' review. Apparently
In the next fiscal year, we have a headline figure for how much day-to-day spending to expect across government. What we don't have is that breakdown.
How much of the total will be health, education, defence and so on? That, in a sense, is the single biggest question the review will set out to answer.
A zero-based review starts from the position that the department will have to justify not just an annual increase (or decrease), but every single pound it spends.
That's tremendously ambitious. And typically zero-based reviews tend to throw out some dramatic changes.
Is that really what Whitehall will provide us with in this review? Almost certainly not.
3. It's the first multi-year review in ages
The last multi-year review happened in the midst of COVID and you have to look back to 2015 for the previous multi year review.
So the fact that this review will set spending totals not just for next fiscal year but for the next three years is no small deal.
4. It's not 'austerity'
The spending totals implied by this spending review are nothing like those implemented by the coalition government between 2010 and 2015.
Day-to-day spending dropped from 21.5% of GDP in 2009/10 to 15% of GDP in 2016/17. By contrast, the spending envelope for this review will see day-to-day spending increasing rather than decreasing in the coming years.
5. It's not a big splurge either
While the totals are indeed due to increase in the coming years, they are not due to increase by all that much.
In each year covered by the 2000 and 2002 comprehensive spending reviews under Gordon Brown, for instance, capital investment grew by 16.3% and 10.6% respectively.
This time around, it's due to increase by just 1.3%. Now, granted, that slightly understates it. Include 2025/26 (not part of this review but still a year of spending determined by this Labour government) and the annual average increase is 3.4%.