The name COLD is an acronym which stands for COsy (Cognitive Systems for Cognitive Assistants) Localization Database. The database represents an effort to provide a large-scale, flexible testing environment for evaluating mainly vision-based localization systems aiming to work on mobile platforms in realistic settings. The COLD database consists of three datasets (COLD-Freiburg, COLD-Ljubljana, COLD-Saarbrücken) acquired at three different indoor laboratory environments located in three different European cities: the Autonomous Intelligent Systems Laboratory at the University of Freiburg, Germany; the Visual Cognitive Systems Laboratory at the University of Ljubljana, Slovenia; and the Language Technology Laboratory at the German Research Center for Artificial Intelligence in Saarbrücken, Germany. The database contains image sequences captured using a regular and omni-directional cameras together with laser range scans and odometry data. The data were recorded using three different mobile robot platforms and the same camera setup, under various weather and illumination conditions (during cloudy weather, sunny weather and at night) over several days. In each of the three labs, the acquisition was performed in several rooms of different functionality. Consequently, the COLD database is an ideal testbed for assessing the robustness of localization and place recognition/categorization algorithms with respect to both categorical and dynamic changes (introduced by illumination variations and human activity).