One issue that arises in regards to measurement is the unresolved problem of how wave function collapse occurs, termed the measurement problem.
Wave function collapse is defined as the process by which a wave function initially in superposition (in multiple states at once) appears to reduce to a single state after interaction with the environment.
In other words, it is the condensation of physical possibilities into a single occurrence. While a physical system can be described by its wave function, a systems wave function cannot be directly observed without initiating wave function collapse. This measurement problem is a central issue in the interpretation quantum mechanics.