This Practice Note considers commercial and legal issues arising from the use of free and open source software including common open source licences such as the GNU General Public Licence, MIT, Apache, BSD and ISC licences. In particular, it looks in detail at the viral or reciprocal effect of open source software in relation to software interactions such as static and dynamic linking. It also considers implications of cloud computing (including SaaS), patents and trade marks in the context of open source.