Dry eye syndrome (DES) is a highly prevalent ocular condition with potential severe consequences for affected patients. DES can be either caused by decreased tear production or increased tear evaporation, both leading to an instable tear film. Despite many efforts, generally accepted methodologies to diagnose, assess the severity and monitor DES are still lacking. Moreover, widely used clinical methods such as tear break up time (BUT), fluorescein staining of the cornea or Schirmer test only poorly reflect patients´ complains. One of the main problems in the diagnosis and treatment of DES is that the most critical component - the tear film itself - is difficult to characterize. The development of new ultra-high resolution optical coherence tomography systems allows now for the direct visualization of the human tear film and for the non-invasive in-vivo measurement of tear film thickness (TFT). The investigators could recently show that this system provides excellent reproducibility and is able to assess even subtle changes in TFT induced by therapeutic interventions. However, to which extent tear film thickness is associated with other standard clinical measures of DES is currently unknown. In the present study, the investigators set out to test the hypothesis that ocular TFT is a new and good surrogate parameter for the assessment of the severity of DES. Consequently, the aim of the study presented in this protocol is to investigate whether and if so, to what extent clinical signs of DES and reported symptoms are reflected in ocular TFT. For this purpose, a cross sectional study in healthy subjects and patients with DES will be performed. This should allow the investigators to more specifically characterize the role of the tear film in DES and to assess whether measurement of TFT with OCT can be a promising surrogate parameter for the diagnosis and the follow up of DES.