Poor balance is the most common source of mortality and reduced quality of life for the millions of people with chronic, neurological disease as well a many elderly people. No cost-effective, portable system is currently available to allow clinicians to objectively quantify postural stepping responses in a clinical environment. The long-term goal of our company, APDM, is to commercialize a state-of-the-art, clinical system to measure mobility disorders using our core technology: the world's most advanced wireless, synchronized, body-worn inertial sensors, the Opals (APDM.com). The objective of this Phase I application is to determine the feasibility of developing a very short balance and gait test related to fall risk using these sensors. The specific aims are: Aim I. To develop algorithms for automatic metrics and a composite 'Fall Risk score' from the Instrumented Stand and Walk Test (ISAW). The objective of Aim I is to develop and test the validity and reliability of a 1-minute balance and gait test for patients with Parkinson's disease. Aim II. To integrate the ISAW into the proprietary user-interface of the Mobility Lab system. The objective of Aim II is to build an innovative, computer user-interface that allows clinicians to quickly, accurately, safely and reliably test 3 aspects of postural control: sway during stance: postural sway, anticipatory postural adjustments, and dynamic balance during gait and turning in a short, 1-minute test. We will initially develop Mobility Lab for patients with Parkinson's disease because they have more falls than any other neurological disorder, but we are confident that this tool will be useful for physical therapists and clinicians and clinical trialists working with any form of mobility disability, such as multiple sclerosis, stroke, head injury, developmental disorders, multisensory deficits, frail elderly, etc. This Phase I application will determine the feasibility of the ISAW test. Our Phase II application will then determine how well the ISAW test predicts future falls, its sensitivity and specificity to mild neurological disease, responsiveness to medication, and safety of the ISAW for a wider range of neurological balance disorders and elderly fallers, in preparation for widespread marketing.