In this paper we define and develop a theory of differentiation in Wiener space C [ 0 , T ] . We then proceed to establish a fundamental theorem of the integral calculus for C [ 0 , T ] . First of all, we show that the derivative of the indefinite Wiener integral exists and equals the integrand functional. Secondly, we show that certain functionals defined on C [ 0 , T ] are equal to the indefinite integral of their Wiener derivative.