Phage therapy is the use of bacteriophages--viruses that use bacteria as their host cells--as biocontrol agents of bacteria. Currently, phage therapy is garnering renewed interest as bacterial resistance to antibiotics becomes widespread. Historically, phage therapy was largely abandoned in the West in the 1940s due to the advent of chemical antibiotics, and the unreliability of phage-based treatments when compared to antibiotics. The choice of phage strain and the methods of phage preparation are now thought to have been critical to the success or failure of phage therapy trials. Insufficiently virulent phages, especially against actual target bacteria, allow bacteria to survive treatment while poorly prepared phage stocks, even if of sufficiently virulent phages, lack the numbers of viable phages required for adequate treatment. In this review we discuss the factors that determine the methods of isolation, analysis, and identification of phage species for phage therapy. We go on to discuss the various methods available for purifying phages as well as considerations of the degree of purification which is sufficient for various applications. Lastly, we review the current practices used to prepare commercial phage therapy products.