A negative pressure wound therapy system comprising: a wound dressing sealable to a patient's skin surrounding a wound and configured to laterally collapse upon application of negative pressure to the wound dressing; and an indicator coupled to the wound dressing and configured to visually indicate an amount by which the wound dressing laterally collapses upon the application of negative pressure wherein the indicator comprises: a first part coupled to a first lateral side of the wound dressing; and a second part coupled to a second lateral side of the wound dressing opposite the first lateral side, wherein the first and second parts (101, 103) of the indicator are slidably connected along a top surface of the wound dressing such that the first part slides under or into the second part when the wound dressing laterally collapses; the first part of the indicator comprising one or more markers on a top surface of the first part, the spacing and arrangement of the one or more markers between a free end and an attachment end of the first part the attachment end being adapted to be mounted to the wound dressing and the free end adapted to remain detached from the wound dressing, being selected and calibrated to correspond to specific pressures to indicate the pressure applied to the wound dressing; and the second part of the indicator comprising a window through which the one or more markers are at least partially visible when the first part slides under or into the second part. a wound dressing sealable to a patient's skin surrounding a wound and configured to laterally collapse upon application of negative pressure to the wound dressing; and an indicator coupled to the wound dressing and configured to visually indicate an amount by which the wound dressing laterally collapses upon the application of negative pressure wherein the indicator comprises: a first part coupled to a first lateral side of the wound dressing; and a second part coupled to a second lateral side of the wound dressing opposite the first lateral side, wherein the first and second parts (101, 103) of the indicator are slidably connected along a top surface of the wound dressing such that the first part slides under or into the second part when the wound dressing laterally collapses; the first part of the indicator comprising one or more markers on a top surface of the first part, the spacing and arrangement of the one or more markers between a free end and an attachment end of the first part the attachment end being adapted to be mounted to the wound dressing and the free end adapted to remain detached from the wound dressing, being selected and calibrated to correspond to specific pressures to indicate the pressure applied to the wound dressing; and the second part of the indicator comprising a window through which the one or more markers are at least partially visible when the first part slides under or into the second part. a first part coupled to a first lateral side of the wound dressing; and a second part coupled to a second lateral side of the wound dressing opposite the first lateral side, wherein the first and second parts (101, 103) of the indicator are slidably connected along a top surface of the wound dressing such that the first part slides under or into the second part when the wound dressing laterally collapses; the first part of the indicator comprising one or more markers on a top surface of the first part, the spacing and arrangement of the one or more markers between a free end and an attachment end of the first part the attachment end being adapted to be mounted to the wound dressing and the free end adapted to remain detached from the wound dressing, being selected and calibrated to correspond to specific pressures to indicate the pressure applied to the wound dressing; and the second part of the indicator comprising a window through which the one or more markers are at least partially visible when the first part slides under or into the second part.