This paper presents results from a literature review of defect characteristics essential for non-destructive testing (NDT). Most of the major NDT methods are included in the study – Penetrant Testing (PT), The study was performed by means of searching in scientific databases, , etc. Mainly, the following It is concluded that for Penetrant testing, the defect geometry, the defect size and the defect. A number of investigations address the relationships between the defect parameters like roller depth, surface defects Also the phenomena of the electrical contacts between the defect surfaces (for a crack) was studied. Defect parameters that are essential to the quality of Penetrant testing are defect position in the object (includes the depth), orientation, size, crack surface roughness, closure and tip radius. This investigation has been focused on those parameters that are not that easy to reconstruct and only briefly discussed the influence on the signal response due to defect position, orientation and size.