Education Minister Ruairi Quinn has urged the 116,527 sitting the leaving and junior certificates to avoid stress.
inety 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.
"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," he said.
There are 55,550 Leaving Certificate students, 57,732 Junior Cert pupils and 3,245 sitting the Leaving Certificate Applied.
Mr Quinn, a qualified architect, has also revealed that his Leaving Cert result from 1964 would not be enough today to get him into UCD 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.
There are 24 schools taking a new examination paper in Junior Cert maths, Paper 2, as education experts push to secure higher average grades through Project Maths. Leaving Cert students in these schools will sit new papers across four of the five strands of maths.
Mr Quinn added: "This is an important reform in the efforts to ensure that Ireland's education system keeps pace with changing needs.
"The experiences in these schools will inform the development of Project Maths as it is rolled out in all schools, a process which began in September 2010."
Jack Keane, president of the Association of Secondary Teachers (Asti), 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."
Other advice for students included maintaining a balanced routine for clarity and stamina during the exams period, plenty of breaks in revision time, a good diet, sleep and fresh air and exercise.
About 15pc of the pupils sitting the Leaving and Junior Cert have some form of examination support, including special educational needs.
Mr Keane added: "Each and every one of our exam students is equally valuable and in Ireland we can be proud of the fact that Irish schools encourage and support students regardless of who they are, where they come from, or their particular abilities and disabilities."