The long-term success of filtering surgery is not dependent on surgical technique alone. The development of the filtering bleb in the postoperative period, in particular with regard to wound healing and subconjunctival scarring, is equally important. Morphologic changes of the developing filtering bleb after trabeculectomy can predict early failure even if the intraocular pressure is still normal. A basic understanding of wound healing processes and histologic changes of the developing filtering bleb are mandatory to interpret correctly the morphologic appearance of the developing filtering bleb. In clinical practice, follow-up of the filtering bleb according to a standardized morphologic classification may help to predict outcome and provide clues for the necessity and timing of further treatment.