Gwyddion is a modular program for SPM (scanning probe microscopy) data visualization and analysis. Primarily it is intended for analysis of height fields obtained by means of scanning probe microscopy techniques (AFM, MFM, STM, SNOM/NSOM), however it can be generally used for any other height field and image analysis It aims to provide multiplatform modular program for 2D data analysis that could be easily extended by modules and plug-ins. Moreover, the status of free software (Gwyddion is Free and Open Source software, covered by license GNU GPL) enables to provide source codes to developers and users, which makes the further program improvement easier. Gwyddion currently supports only a limited set of data file formats, but there are many data formats used in the field of Scanning Probe Microsopy.