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This paper discusses maintenance considerations and field experience with the Bell System's No. 1 Electronic Switching System (No. 1 ESS). A brief introduction to the ESS includes a comparison with computers and a history of ESS development, which is followed by a discussion, in some technical depth, of particular areas of ESS maintenance design and practice. One example is the use of dictionaries to isolate equipment faults. The effectiveness of the dictionaries is also discussed. Performance data of several types are presented for the central offices already in service.