This book is prepared not only for the students of pure and applied Mathematics but also for the students of all discipline. Mathematics is an essential and mandatory subject for all the students from their early stages. This book is also helpful for them because elementary definitions and properties on real numbers have been discussed here. I have tried to make the exposition of the fundamental principles clearly as well as concise without going unnecessary details and at the same time an attempt has been made to make the treatment as much rigorous and up-to-date as is possible within the scope of this elementary book.I have made different chapters for the discussion of each different topics clearly.A good number of numerical examples have been given with different definitions so that interested reader can understand easily the main theme.Solved problems have been discussed very easily as much as possible.At the eleventh hour of this book, I have displayed how a real number can take several identifications on different sets which remove all confusions regarding a real number.I think a good book is a good friend if one uses this carefully and my opinion is, this book is that one.