With linear differential equations, we can distinguish two types: there are those where all coefficients are constant, and those where this is not the case, where some coefficients are functions in t. One immediately suspects that the solution finding for those with non-constant coefficients is generally more difficult. Indeed, there is no longer a general method for finding solutions when the order is greater than or equal to 2. All the more astonishing is that all linear differential equations with constant coefficients can generally be solved by a clear scheme (provided the disturbance function does not disturb too much). We deal with this in the present chapter.