Tentative parameters on what constitutes a perfect membrane system are advanced. Liquid applied, seamless, elastomeric, film forming membranes are seen as meeting most of the requirements that would constitute approaching the perfect membrane system, and properties of these systems are given. The effectiveness of the liquid applied membranes is seen to be directly related to the details of construction, with suggested membrane details for maximum waterproof results. Other areas covered are the use of the liquid applied membranes as surface seals, with related components and accessories; the sealing of horizontal joints in conjunction with liquid applied membrane systems; and the sealing of building deck surfaces. This paper concludes with a definitive limit placed on manufacturers of liquid applied membrane material in the area of responsibility and valid guarantees.