ASM (Aquifer Simulation Model) is a complete implementation of a 2-D groundwater model for use on a PC under MS-Windows 3.1x/95/98/NT (tm). It was originally developed as a tool for the education of students in hydrogeology, civil and environmental engineering. The first version of ASM was published in 1989 and ran under MS-DOS. Since that time ASM has been continually enhanced and improved. This users manual documents the latest and most powerful version 6.0 of ASM which runs under the Microsoft Windows(tm) operating system. The most notable change in this version is the use of a new, friendly graphical user interface which replaces the character oriented interfaces of previous versions. At the same time, ASM-6.0 allows the manipulation of larger model grids which makes it suitable for professional use. It is hoped that this very user-friendly implementation of a groundwater model on Windows(tm) PCs will lower the threshold which inhibits the widespread use of computer based groundwater models. Its ease of use allows getting acquainted with the properties and peculiarities of groundwater flow and transport paths by experimenting with the models by changing relevant parameters. Readers unfamiliar with the theoretical background of groundwater hydraulics and groundwater modelling are referred to the textbooks listed in the references section of this manual.