1. A metrology system comprising: an illumination source configured to provide an amount of illumination light to one or more metrology targets; a detector configured to receive an amount of collected light from the one or more metrology targets in response to the amount of illumination light; one or more computer systems configured to: receive an amount of measurement data associated with the detected light; and determine one or more parameters of a measurement model of the one or more metrology targets based on a fitting of the measurement model to the amount of measurement data, wherein the measurement model includes a first re-useable, parametric model of a first sub-structure of the one or more metrology targets; and a metrology model building tool comprising computer-readable instructions stored on a non-transitory, computer-readable medium, the computer-readable instructions comprising: code for causing the one or more computer systems to receive an indication of a selection of the first re-useable, parametric model by a first user to describe at least a portion of the one or more metrology targets, wherein the first re-useable, parametric model includes multiple geometric elements and is fully defined by a first set of independent parameter values; and code for causing the one or more computer systems to receive an indication of a selection of the first set of independent parameter values.