Langmuir-Blodgett (LB) films are formed by the successive deposition of a series of monolayers of one or more types of amphiphilic molecules initially spread at the interface between water and air. They usually consist of a regular planar array of molecular layers having a well defined and predetermined thickness. Their properties have made them of interest to physicists, electronic engineers, chemists and biologists. This review is mainly addressed to physicists and device engineers but, since the topic is of an interdisciplinary nature, it is hoped that it may also be of use to research workers in other disciplines. The early development and essential technology of the subject are discussed and key papers are referred to. However, the review is mainly devoted to the large amount of recent work in the field. The application of LB films to the study of dye molecules and their interactions is not dealt with in any detail as this topic has been well reviewed elsewhere and leads, in any case, to the general study of dye molecules which is beyond the scope of this review. A wide variety of different types of amphiphilic molecule are discussed ranging from simple carboxylic acids via complex heterocyclic compounds to polymers. The various physical techniques used to determine the crystallographic structure of the films together with the influence of deposition methods on this structure are discussed. The study of the electric and optical properties of various LB films is described both in relation to the physical information which may be gained from these studies and also in respect of possible technical applications. The difficulties inherent in attempting to obtain good threedimensional crystallographic order in films are pointed out together with possible methods of overcoming these difficulties.