The IAEA has conducted a Coordinated Research Project (CRP) on Intercomparison of Techniques for HWR Pressure Tube Inspection and Diagnostics. This publication reports on the first phase of the CRP dealing with flaw characterization by in-situ non-destructive examination (NDE) techniques. The participating laboratories prepared pressure tube (PT) samples containing artificial hidden flaws resembling real defects of concern. All samples were then inspected from the inside surface, as in real conditions. The originating laboratories subsequently analysed the sample inspection reports from investigating laboratories and compared the NDE flaw sizing estimates with their true values. Thus, the most successful NDE methods for detection, location and sizing of various types of flaws in PTs were identified. This publication details how the research was conducted and documents results from the flaw detection and characterization activities.