This thesis concerns the spectral theory of the Laplacian on Riemann surfaces of finite type, with emphasis on the quotients of the upper half plane by congruence subgroups. In a first part we show, following Otal, that on a Riemann surface M of genus g with n punctures there are at most 2g - 2 + n eigenvalues $\lambda$ with $\lambda$ $\leq$ 1/4. In a second part, we focus on arithmetic surfaces. This subject is treated by Maass in a paper that is difficult to read. We work out some examples of his construction of Maass forms.