There are described methods and apparatus for determining the condition of a barrier in a well infrastructure, for example. In some examples, sensor data that has been derived from a sensor arrangement at a barrier positioned at a location in a well infrastructure in received. That sensor data may be associated with measured conditions at the barrier, or the like. Composition data derived from measurements of fluid composition within the well may also be received. Such composition data may be indicative of the location at which fluid has been in the well. Analyzing such received sensor data and composition data may help determine the condition of the barrier. In some examples, there is described well apparatus comprising a barrier for zonal isolation, a sensor arrangement configured to monitor conditions at the location of the barrier; and one or more tracer elements configured to interact with fluid at the location of the barrier so as to impart identifiable properties to the composition of fluid.