Though negative numbers have been used in China and India for a long time, it has been much more difficult to introduce them in Europe. The first references to this idea refer to the XVth century, but the existence of such quantities has not been approved before the middle of the XIXth century. Yet they were not accepted as numbers but only negative values. The most notable scientists at that time showed great difficulties to accept and understand negative numbers. Then, it is hard to believe that secondary school students can easily handle them and their operations. In a first part, we have analysed how the relative numbers and addition and subtraction are introduced in the textbooks of grade 7 (students of 12-13 years old). Then, we have chosen some activities we proposed in our classes in order to introduce relative numbers. We have particularly focused on the different meaning of sign « + » or « -« .