Book excerpt: This Guide assesses the key clinimetric attributes in the assessment of Parkinson's Disease (PD), with the intention to offer rapid and pragmatic information on the most relevant scales used in PD. Parkinson’s disease affects approximately 4 million people globally and is most commonly seen in people over the age of 50. The disease is a progressive disorder of the nervous system, and presents a number of movement and cognitive symptoms, thereby greatly affecting a patients quality of life. The use of scales for assessment in neurological disorders such as PD arises from the need to quantify disorders and states (such constructs as disability, symptoms, quality of life). Assessment scales are often categorised into two categories: generic (i.e. those scales usable in any health condition), and specific (i.e. scales developed for exclusive use in PD). They can have a variety of components: single-item and multi-item or composite scale; unidimensional and multidimensional; and as disease-centered and patient-centered measures. The creation and validation of scales is complex, with scales undergoing numerous studies to assess criteria such as acceptability, reliability, and responsiveness. In the process of validation of a scale the following attributes should be tested to ascertain whether a scale is an effective instrument of measurement.