"1. Introduction Non-motor symptoms (NMS) are a crucial component of Parkinson's disease (PD) and the burden of the range of NMS that occurs in PD is one of the main determinants of quality of life [1]. In 1817, Dr James Parkinson already described several NMS in his "shaking palsy", the condition that would later be named after him, including pain, constipation and sleep disturbances [2]. Nonetheless, after a long period of inertia, over the last 20 to 25 years the interest in NMS has increased and evidence suggests that the overall burden of NMS dominates and can influence the risk of developing motor parkinsonism in the premotor stage of PD while being a driving factor for quality of life [3]. The identification of NMS in PD has been greatly aided by the development of specific tools, such as the NMS questionnaire (NMSQ) in 2006 [4] and the NMS Scale (NMSS) in 2007 [5]. The NMSS, which was based on the previously validated NMSQ, addresses a wide range of NMS in PD which are grouped into nine domains, measured over the period of the last month and quantified by severity and frequency (NMS symptomatic burden) [5]. Recently, a new version of the NMSS, the Movement Disorder society - Non-motor symptoms scale (MDS-NMS), has been published which will further aid in the identification and quantification on NMS burden of PD [6]. In this chapter, we will describe the phenomenology of NMS in PD, including non-motor fluctuations (NMF), and how to measure these symptoms"--