There are many ways to manage concurrency:  with threads, co-routines,
continuations, generators, ... and events.  A page on [Event programming and why it is relevant to Tcl/Tk programming]
explains some of the background behind Tcl's emphasis of the latter.

Events are a relatively "safe" programming model, particularly when compared to threading.  [John Ousterhout]'s influential "Why Threads Are A Bad Idea" is available only as PowerPoint slides [http://home.pacbell.net/ouster/threads.ppt].

Notice that C# makes events first-class objects.  This is consistent with Anders Hejlsberg's earlier work with Delphi ...
















----
This Wiki has several other pages [http://www.purl.org/mini/tcl/2?event] on event-oriented programming techniques.