This presentation will summarise the development of railway EMC standards from the early RIA documents to the latest version of the EN50121 series of standards. It will describe the scope and application of each part and sub-part. The presentation will consider changes that take place, both at a standards level and a directive level; issues that manufacturers need to be aware of, so ensuring that the remain compliant. The principle of using the dap analysis techniques will be described to address cases where equipment has not been assessed against the relevant EN50121-X standard, but to another European harmonised standard. It will show that despite the standard numbers not being the same, there may well be considerable commonality between the tests performed and therefore only a small amount of remedial work may be required. (24 pages)