The "original" Bolshoi Theater is currently under renovation, and is expected to remain so, at least until sometime in 2009.  According to a recent (as of February 2008) Reuters news article, the people working on the renovation have said that they expect to have the theater up and running by November 2009.  The word "original" is in quotes above, because even that building is not really the original Bolshoi Theater.  The original building was destroyed by fire at least once during the 19th Century, and was, perhaps destroyed and rebuilt multiple times.  At any rate, even after November 2009, you should check to make sure that the theater has re-opened before assuming that you will be in the original theater building.  Yes, you can still see the "Bolshoi Ballet", even before the renovation is finished, but you won't be seeing it in the original theater building.  You will be viewing it on the "New Stage", which is where performances are being held during the renovation period.  The New Stage is located just across the street from the original theater.  Not that the location should be all that important to a ballet lover - this is, after all, one of the finest ballet troupes in the world.  The point is, however, that a lot of folks who really don't care that much for ballet still want to see the ballet at the Bolshoi, just because it is such a famous place - not really because they love ballet.  That's why this distinction is important.  If you want to go to the Bolshoi mainly to be "at the Bolshoi," then you won't be able to do that until the renovation is finished, at least, not if it is important to you to be in the "real" or "original" Bolshoi.   On the New Stage, however, the quality of the theater and the performances is just as good as in the original building.  This from someone who has seen performances in both buildings.

 You may order tickets online at www.bolshoi.ru  This site has menus in both Russian and English, and is very easy to use.  You can pre-purchase and pick tickets up at the box office.  Just follow the instructions that you will be given at the time of purchase.

Due to high demand, tickets to the most popular performances often sell out early, especially during the summer (May-August), since summer ballet performances are very limited (the "season" starts in September).  If you are traveling to Moscow, it is strongly recommended to buy tickets online in advance. Getting tickets on the spot after your arrival to Moscow may be difficult and/or expensive.