DNA microarray assays represent the first widely used application that attempts to build upon the information provided by genome projects in the study of biological questions. One of the greatest challenges with working with microarrays is collecting, managing, and analyzing data. Although several commercial and noncommercial solutions exist, there is a growing body of freely available, open source software that allows users to analyze data using a host of existing techniques and to develop their own and integrate them within the system. Here we review three of the most widely used and comprehensive systems, the statistical analysis tools written in R through the Bioconductor project ( http://www.bioconductor.org ), the Java^®-based TM4 software system available from The Institute for Genomic Research ( http://www.tigr.org/software ), and BASE, the Web-based system developed at Lund University ( http://base.thep.lu.se ).