The aim of the paper is to provide an expert opinion on the issue of improvised explosive devices, their characterisation, classification, and construction. Their explosive effect is described in the next step. Major part of the paper is focused on the proposed methodology for estimation of explosive charge size, which was used in terrorist attack. The methodology is based on the analysis of a damage level of buildings in the surrounding of the explosion. On the base of a damage level of buildings, the weight and type of the used explosive is estimated.