This book reproduces work written for a PhD thesis. It introduces new theoretical methods for the solution of protein structures through the processing and interpretation of crystallographic data coming from high-resolution X-ray diffraction experiments. Only data for the native protein are used, i.e. the procedure here explained and tested implies ab initio solution. This method has been implemented in a computer package, ACORN, whose main underlying concept is that a proper structural fragment, equivalent to 5-10% of the structure being studied, can give rise to enough information to start a process of Fourier recycling and density modification in real space, leading to the solution of the structure. Tests performed on RNAse AP1 in this thesis and on other structures in another context, have proved that ACORN is a robust procedure and that it can be adapted to many different situations.