This thesis examines the concepts of usability and vulnerability. Through the creation of anew perspective, the ‘subtask perspective’, the concept of usability is transferred from the field of human-computer interaction to a perspective more concerned with the safe and reliable performance of systems. This thesis aims to argue why this ‘subtask perspective’ can positively affect a systems ability to detect and handle vulnerability. The perspective is based on ideas from the field of human-computer interaction, but adopts the system perspective (Snook, 2000) and the concept of vulnerability (Wackers, 2006) from the field of science, society and technology studies (STS). The main argument in this thesis is that the concept of vulnerability can fruitfully be studied from the ‘subtask perspective’ created. While a system’s vulnerability refers to the state the system is in, the usability of a system’s subtasks refers to the systems ability to perform reliably.