A pragmatic approach to interference in NMR and MRI systems is presented with the objective of raising the awareness of problems associated with larger installations. The basic premise is that an installation is a system composed of interconnected sub-systems, each with its own idiosyncracies. Consequently all problems should be considered from this view-point. Practical warnings and solutions are given along with simple mathematical equations to permit one to calculate some interference effects. Most problems are associated with RF and gradient sub-systems, so the main content deals with degradation of the NMR signal by noise and coherent RF interference and with the interaction of sub-systems. Attention is drawn to the correct identification of the source of a problem, and the importance of a well engineered solution. 'Quick-fix repairs' can lead to very baffling problems when the system is reconfigured for a different experiment.