This paper presents an approach to increase the understanding of errors and alignments in interferometrical contouring. In order to achieve a fully determined interferometrical set-up with a controllable alignment status, real existing interferometers are simulated with the help of a software package called the Virtual Interferometer (VI). It consists of a raytracing module, a measurement simulator for phase shifting evaluation and a statistics module to simulate the environmental aspects of interferometrical contouring (e.g. misalignments, surface errors of the optical components, phase shift errors). Additionally, a further software package for controlling the corresponding real interferometrical systems and a module for addressing two separate positioning systems each having 5 degrees of freedom was developed. With the help of the VI, two closely related aspects of interferometrical shape testing were examined. First, work in the field of testing aspheric surfaces with the use of Multiple Wavelength Interferometry (MWI) will be presented. New experimental results are introduced and the effect of aberrations due to strong surface slopes is discussed. Second, nulltests are presented that allow the measurement of aspheric mirrors like paraboloids and hyperboloids. In this application, especially the alignment of the complex interferometrical setup with its up to ten independent parameters was the main point of investigation. Both applications were analyzed with the VI in order to optimize the measurement methods with regard to alignment optimization and correction of error inducing aberrations. Then the optimized methods were applied in the real interferometrical systems for measuring various aspheric surfaces. Finally, future application for the VI will be discussed shortly.