Modern smart devices such as pressure transmitters, controllers and valve actuators provide many key advantages but there are well-known difficulties in providing evidence to support the associated safety justifications. This paper reminds the reader of these difficulties but focuses on the need for an “As Low as Reasonably Practicable” approach and hence the requirement to use expert engineering judgement to weigh up the advantages/disadvantages of using a smart device against other possible options. The paper discusses the use of engineering judgement in safety justifications for installing smart devices and highlights that there are more than just software faults to consider. Issues covered include: allocation of ‘best estimate’ reliability data for use in Probabilistic Safety Assessments, ‘proven in use’ arguments and the importance of understanding the wider safety case picture.