This page provides an introduction to modifications and repairs for weapons and armor in S.T.A.L.K.E.R.: Call of Pripyat.
Modifications are alterations to weapons and armor that improve their characteristics and add new features. They are performed by technicians in exchange for money. Some modifications requires special tools that you must acquire for the technician before he can apply the modification.
There are three technicians in the game that can provide modifications and perform repairs, and two that only do repairs:
Each weapon and armor usually has one or more chains of modifications with up to three individual links. For most chains, to get the last modification you must first buy a modification in each of the previous links. Some links have an option of two modifications in which case you can buy one, but not both, of them; this means that you usually cannot buy every single modification for a weapon or armor since some modifications rule others out. This is demonstrated in the following example: