MDPP is a fully featured general-purpose image processing package primarily written to support research in structural biology using data gathered by electron microscopy. It has focused on the analysis of images using Fourier techniques, particularly periodic plane-layer and helical structures, but has extensive tools for other processing options (e.g., point-counting, image quantitation, DNA sequencing, and display). Three-dimensional reconstruction methods are supported, including iterative deconvolution schemes for light micrographs. The basic program is command line-driven, but the user can choose to write sophisticated command scripts and use a menu-driven full-screen or Motif interface to call them. Color images are supported with both color palette and RGB options. Extensive documentation is accessible from within the program as online HELP and also as HTML. Care has been taken to support interfaces to other image processing packages, e.g., MacIntosh applications via the TIFF image format, and other EM-targeted image processing packages. The paper provides an overview of the program, its design, and implementation and outlines future plans for software development. Some general issues concerning image processing program design are discussed. (C) 1996 Academic Press, Inc.