Abstract This paper is about instrument localization in the operating room during surgeries. The main purpose of the proposed system is the automatic determination of the possible instrument usage information for automatic surgical workflow re-cording. The approach is based on Radio Frequency Identification (RFID), sensor fusion methods and Hidden Markov models (HMM). The Results show high accuracy for instrument localization and demonstrate the ability of the system to replace manually recorded instrument usage information. 1 Introduction Since the number of medical devices in the operating room is increasing and the handling of them is getting more complex, there is an upcoming demand for automatic as-sistance systems. Major requirement for timely assistance is to recognize the current surgical activity of the interven-tion. The recognition of the current activity enables the sit-uation dependent assistance of the surgeon. Previous works dealt with the manual recording of workflow activi-ties [4]. The goal of current research is to automate the process of workflow recording. A