More than 700,000 teenagers across England, Wales and Northern Ireland are receiving their GCSE results.
In England, there have been major changes to the grading system over the past few years.
What are the new grades?
The 9-1 grading scheme was brought in alongside a new GCSE curriculum in England.
The highest grade is 9, while 1 is the lowest, not including a U (ungraded).
Three number grades - 9, 8 and 7 - correspond to the old-style top grades of A* and A - this is designed to give more differentiation at the top end.
Exams watchdog Ofqual says fewer grade 9s are awarded than A*s, and that anyone who gets a 9 has "performed exceptionally".
A 4 is broadly equivalent to a C grade, although Ofqual warns against "direct comparisons and overly simplistic descriptions".
It says that, broadly, the same proportion of teenagers get a 4 or above as used to get a grade C or above.
This year could see a big spike in the number of higher grades awarded, as GCSEs will be based on teachers' predicted grades rather than through exams sat by pupils. This is due to the coronavirus pandemic, which led to all summer exams being cancelled.
When were the new grades brought in?
The numerical grading was phased over four years, starting with the core compulsory subjects - maths and English GCSEs - in 2017.
Most of the main subjects switched over in 2018, including the humanities, sciences and most modern languages.
This year sees the last few remaining subjects go over to numerical grading - among them Biblical Hebrew, Persian, Portuguese and Turkish.
Strong pass and standard pass - what's all that about?
It's confusing, but there are two pass marks - 4 is a standard pass and 5 is a strong pass.
This means that a candidate who gets nine grade-4s has, technically, passed all their exams.
The reality is that schools are pushing pupils for at least a Grade 5 and most sixth forms are looking for students with strong passes.
Why were the grades changed?
The numerical grading scheme is part of a curriculum introduced in England's schools in 2014 by then Education Secretary Michael Gove.
GCSE courses now include much less coursework than before, with grades in almost all subjects depending on exams.
Courses are designed to be more challenging, with exams taken after two years of study rather than in modules with exams along the way.
What's happening in Northern Ireland?
There are some changes in Northern Ireland too.
The NI Council for Curriculum, Examinations and Assessment (CCEA) has introduced a new nine-category grade scale - A* to G, including a C*.
This is the second year results will be graded under this scheme.
But students may also get results with grades 9-1 if they take exams set by English boards.
What's happening in Wales?
The Welsh government introduced new and revised GCSE courses in September 2015.
The most significant changes were in English language, Welsh language and mathematics.
They are different from the English exam in that they have retained the letter-based grading structure A*- G.
And what about Scotland?
Scotland has its own system of public examinations - Nationals and Highers.
Nationals replaced the old Standard Grades in 2014, and new Higher exams were introduced in 2015.