S2S

In mathematics, S2S is the monadic second order theory of the infinite complete binary tree.

S2S may also refer to:
 * Server-to-server, protocol exchange between servers
 * Site-to-site VPN
 * S2S Pte Ltd, a Japanese record label
 * Ski to Sea Race, a race in Whatcom County, Washington
 * Sister2Sister, Christine and Sharon Muscat, Maltese-Australian singers
 * Sales to Support, type of transfer on Call Center lines, Sales to Technical Support Transfer
 * In woodworking or lumber terms S2S= surfaced two sides, while S3S = surfaced three sides and s4s = surfaced four sides