1. A method, for use by a first sync agent ( 11 b 12 b ) operative in association with a first data store ( 11 c 12 c ) and by a second sync agent ( 11 b 12 b ) operative in association with a second data store ( 11 c 12 c ), by which the first data store ( 11 c 12 c ) is synchronized with the second data store ( 11 c 12 c ), the data stores ( 11 c 12 c ) each being used for storing data as data units in folders by a first and second respective application ( 11 a 12 a ) distinct from the first and second sync agents ( 11 b 12 b ), the folders in combination defining a data structure, the method comprising: a step ( 31 a ) in which the first and second sync agents ( 11 b 12 b ) establish a transport connection ( 14 ) for enabling communication between the first and second sync agents ( 11 b 12 b ); and a step ( 31 e 31 j ) in which the second sync agent ( 11 b 12 b ) communicates via the transport connection ( 14 ) a message ( 21 ) to the first sync agent ( 11 b 12 b ), the message ( 21 ) expressed using a markup language and having at least one data identification element ( 26 27 ); wherein information about a change in the data structure of the first or second data store ( 11 c 12 c ) is transmitted in said message; and further wherein said information about a change in the data structure of the first or second data store ( 11 c 12 c ) is placed in the at least one data identification element ( 26 27 ).