The Supplementary International Search is an optional additional search carried out by a second International Search Authority (SISA) to identify published documents (prior art) that may not have been found by the first International Search Authority (ISA - the Authority that conducted the main International Search) due the increasing diversity of prior art in different languages and different technical fields.

A growing problem in finding relevant prior art is that the range of languages in which original technical disclosures are made is constantly increasing. The machine translation can be used to a certain extent to translate disclosures, though this is not as reliable as having access to the results of a search carried out by an examiner who understands the original language. Therefore, given that no single ISA is capable of searching the whole relevant documentation in its original language, supplementary search permits the applicant to have a search carried out by an ISA which specializes in documents in a particular language or languages. For example, if the main international search has been carried out by the European Patent Office, but there are a large number of patent documents in Russian which cover the technology relevant to the international application, it may be useful to request that the Federal Service for Intellectual Property, Patents and Trademarks (Rospatent) carry out a supplementary search. 

One advantage of the supplementary search system is that, unlike the requirements for the choice of ISA, the choice of SISA is not limited to a specific International Authority (or Authorities), therefore the applicant is free to request any of the International Authorities which carry out supplementary searches. Consequently, the applicant has the possibility of obtaining one or more supplementary searches carried out by a search authority other than the international search authority that carries out the main international search.

Even though the main international search is of high quality and covers not only the PCT minimum documentation but also all the additional documentation which would be searched by the International Searching Authority (ISA) in its role as a national Office, a supplementary search should further reduce the risk of new prior art being cited in the national phase, allowing the applicant to be better informed about the prior art and, therefore, the chances of obtaining a patent before you incur national phase expenses. 

The supplementary could be, strategically, considered after the analysis of the results of the main international search, considering the commercial importance of the particular application and the amount of prior art in the particular technical field which is known to be published in a language in which the main ISA is not skilled. This additional search is a way of obtaining extra information about the prior art in a particular language, where the additional expense in the international phase is worthwhile.