Book excerpt: Geosynthetics can, and have, played a pivotal role in providing the primary functions of filtration, drainage and erosion control. Within each category this book counterpoints the design, testing and performance of the various materials against one another. The facilitation of filtration by a number of different woven and non–woven geotextiles is discussed. Design is centred around a balance between open voids [for adequate permeability] and closed voids [for proper soil retention]. This balance is compromised by long term clogging or soil loss from either the upstream soil particles or by the nature of the permeating fluid. This is a major focal area of the book. One solution to excessive filter clogging is to open up the geotextile's voids and allow sediments and micro–organisms in the permeating fluid to pass through. The challenge then becomes the design and potential clogging of the drain. The drainage aspect of geosynthetics is the second focal area. Erosion control is closely related to both filtration and drainage. The tremendous design problems, and equally large repair problems on all types of facilities, are addressed. Highway slopes, earth dams, landfill covers and solid waste daily covers are a few common situations.