Nearly 700,000 pupils in England, Wales and Northern Ireland are receiving their GCSE results, as schools face tougher targets on pass rates.
Some 40% of a school's pupils are now expected to get five A* to C grade GCSEs - up from 35% last year.
And changes are under way to stop grade inflation and ensure pupils have a thorough grasp of spelling and grammar.
The proportion of entries awarded top grades has risen every year since GCSE exams were first taken in 1988.
Graham Stuart, the Conservative chairman of the education select committee, said: "The standard of performance is better than it's ever been, the teaching is better and the children are cleverer than ever before.
"The key question is, are (GCSEs) fit for purpose today?"
Last year 70% of entries to GCSE exams, which replaced O-levels and CSEs, were awarded between an A* and a C grade.
A total of 658,000 16-year-olds across England, Wales and Northern Ireland will receive their results on Thursday. A further 547,000 candidates, many of whom will have sat exams a year early or as adults, will also receive their grades.
Changes to GCSEs
Modular GCSEs are being dropped in England, so that pupils starting GCSE courses this September will have to sit all their exams at the end of the course.
GCSE exams sat in 2014 in English literature, geography, history and religious education will also be assessed for spelling, punctuation and grammar.
Schools in Wales and Northern Ireland will continue to be able to opt for modular GCSEs.
Changes which could see a return of O-Level-style exams are heralded further down the line.
In an attempt to address concerns of "dumbing down" and ensure results are comparable, the exams regulator for England, Ofqual, has told exam boards they will have to justify any results that are notably different to previous years.