Building automation systems (BAS) and building energy management systems (BEMS) have been used in modern edifices for the last 2 decades. These systems measure, process, and monitor a huge amount of data to operate the building and systems more or less properly. Often, the data is only used to signal failures or break-downs of systems or components. Further information of the data to analyze and diagnose the building operation is not used due to the lack of analysis methods and tools. Within the Operation Diagnostics, different visualization techniques are applied to display the information that is hidden in the recorded operation data of buildings and systems. Different diagrams and plot types are used to visualize data. In the first part of the presentation, the general approach of using different visualization forms to display operation data will be discussed. The second part describes a practical example for the performance of operation diagnostics by means of an air handling unit (AHU).