In this chapter, we study functions, called multiplicative functions, that are defined on integers. These functions have the property that their value at the product of two relatively prime integers is equal to the product of the value of the functions at these integers. We start by proving several theorems about multiplicative functions that we will use later. We then study special functions and prove that the Euler \(\phi\)-function that was seen before is actually multiplicative. We also define the sum of divisors and the number of divisors functions. Later define the Mobius function which investigate integers in terms of their prime decomposition. The summatory function of a given function takes the sum of the values of \(f\) at the divisors of a given integer \(n\). We then determine the Mobius inversion of this function which writes the values of \(f\) in terms of the values of its summatory function. We end this chapter by presenting integers with interesting properties and prove some of their properties.