The literature on runtime “verification” (the activity of monitoring a running process to ensure compliance with a given specification) offers many different definitions of “monitorability” for properties. In a linear-time temporal setting, properties take the form of sets of behaviours (infinite sequences of states). In this talk, I will arrange the various monitorability classes found in the literature into a hierarchy along with well-known classes of properties such as safety and guarantee properties. I then give each of these monitorability classes neat topological characterisations within the topology induced by the "guarantee kernel" interior operator. Lastly, I show that the metric space used by Alpern and Schneider coincides with our topology.