The majority of instrumentation and controls (I&C) used in commercial nuclear power plants (NPP) were originally designed using older 1970s era analog-based technology. As the plants continue to age, analog-based controls have become obsolete and are no longer supported by original equipment manufacturers (OEMs). As technology evolved, OEMs have transitioned to modern software/firmware-based digital controls. This has left NPP owners with few alternatives other than to upgrade their systems using modern digital I&C technology. Digital systems present a different set of issues than analog-based systems. Among them include their vulnerability to malicious cyber security attacks that could compromise the integrity of the I&C system, and the challenge of protecting them from such attacks. The United States Nuclear Regulatory Commission (NRC), Nuclear Energy Institute (NEI), and the cyber security working group are working to address the cyber security issues in the digital systems. The NRC published a set of cyber security standards for the industry to follow. NEI published guidelines to implement the NRC cyber security standards. The NRC has endorsed NEI published guidelines. The nuclear industry is following these guidelines to implement cyber security to comply with NRC and Department of Energy regulations. Prior to terrorist’s attacks of 9/11, the nuclear industry gave little consideration to cyber security when implementing digital upgrades to I&C systems at NPPs. In general, cyber security was considered only a function of information technology (IT) and not a plant engineering function. Most commercial grade cyber security products are oriented to IT specific applications and have not been developed to protect modern digital I&C applications used in NPPs. Back fitting cyber security to address plant I&C systems without impeding functionality presents a huge challenge. Digital equipment is relied on now more than ever for plant operation, and as the use of digital equipment increases, so does the frequency of cyber-attack and risk of compromise. The nuclear power industry had to overcome multiple challenges during the creation and application of regulatory requirements and standards to address cyber security risks and vulnerabilities when applying IT based cyber security technology to NPPs digital instrumentation and controls. Among these include: 1) clarifying/translating the IT based cyber security standards to nuclear industry; 2) realizing the full scope of critical digital components; 3) documenting the configuration of digital plant equipment; 4) employing alternate methods to achieve compliance; 5) training/hiring staff to adequately implement the program; 6) obtaining organizational concurrence. This paper addresses the challenges that applied to the phased in approach the nuclear power industry faced to implement cyber security requirements applicable to digital I&C systems and components.