Education Minister Ruairi Quinn has sent a good luck message to the 116,527 students from about 160 countries who have taken to exam halls for the Leaving and Junior Certificate.
Ninety different subjects will be tested over the next few weeks, with results due out in August, and exams will also be answered in 15 languages other than English such as Polish, Lithuanian and Romanian.
Mr Quinn said: "After many long hours of study and preparation, students finally get the opportunity to demonstrate their knowledge, understanding and skills across a range of areas. This marks an important milestone in their lives."
There are 55,550 Leaving Certificate students, 57,732 Junior Certificate pupils and 3,245 sitting the Leaving Certificate Applied.
Mr Quinn, a qualified architect, has also revealed that his Leaving Certificate result from 1964 would not be enough today to get him into University College Dublin for the degree. And he urged students not to be downhearted if exams do not go the way they hoped.
"I hope students will be able to keep the exams in perspective and realise that there are many ways and opportunities for you to continue your education after the Leaving Certificate. I know your hard work will pay off and I wish you every success," he said.
The first exam in the Leaving Certificate is English. There are 24 schools taking a new examination paper in Junior Certificate Maths, Paper 2, as education experts push to secure higher average grades through Project Maths. Leaving Certificate students in these schools will sit new papers across four of the five strands of maths.
Jack Keane, president of the Association of Secondary Teachers, urged pupils to try to keep anxiety over exams to a minimum.
"While it is perfectly normal to feel some anxiety, it is really important to keep things in perspective over the coming weeks," he said. "No one exam is going to make or break you as a person and, while exams are important, they are not everything. Your worth as a person is not tested by any examination.
"My advice to students is to take each day as it comes and avoid looking back on an exam once you have finished, or catastrophising about an exam you have yet to sit."