Computer based digital instrumentation and control (I&C) systems are widely used in non-nuclear industry safety systems. However, there are uncertainties and challenges in the safety assessment of such systems in the nuclear industry. This publication describes potential sources of common cause failures (CCF) in computer based safety I&C systems and presents approaches on how to evaluate the vulnerability of digital I&C systems to CCFs. The intended audience of this publication includes nuclear utility personnel, vendors, regulatory authorities and others involved in the design and implementation of I&C systems in nuclear power plants.