  In the year 1905 Albert Einstein published four papers that raised him to a
giant in the history of science of all times. These works encompass the photon
hypothesis (for which he obtained the Nobel prize in 1921), his first two
papers on (special) relativity theory and, of course, his first paper on
Brownian motion, entitled "\"Uber die von der molekularkinetischen Theorie der
W\"arme geforderte Bewegung von in ruhenden Fl\"ussigkeiten suspendierten
Teilchen'' (submitted on May 11, 1905). Thanks to Einstein intuition, the
phenomenon observed by the Scottish botanist Rober Brown in 1827 - a little
more than a naturalist's curiosity - becomes the keystone of a fully
probabilistic formulation of statistical mechanics and a well-established
subject of physical investigation which we celebrate in this Focus issue
entitled - for this reason - : ``100 Years of Brownian Motion''.
