Assistive technologies to support persons with dementia and their carers are used increasingly often. However, little is known about the effectiveness of most assistive devices. Much technology is put on the market without having been properly tested with potential end-users. To increase the chance that an assistive device is well accepted and useful for the target group, it is important, especially in the case of disabled persons, to involve potential users in the development process and to evaluate the impact of using the device on them before implementing it in the daily care and support. When evaluating the impact, decisions have to be made regarding the selection of measuring instruments. Important considerations in the selection process are the underlying domains to be addressed by the assistive technology, the target group and the availability of standardized instruments with good psychometric properties. In this chapter the COGKNOW project is used as a case example to explain how the impact of cognitive prosthetics on the daily lives of people with dementia and their carers can be measured. In COGKNOW a cognitive prosthetic device is being developed to improve the quality of life and autonomy of persons with dementia and to help them to remember and remind, to have social contact, to perform daily activities and to enhance feelings of safety. For all these areas, potential measuring instruments are described. Besides (standardized) measuring instruments, other data collection methods are used as well, such as semi-structured interviews and observations, diaries and in situ measurement. Within the COGKNOW project a first uncontrolled small-scale impact measurement takes place during the development process of the assistive device. However, it is recommended to perform a larger randomized controlled study as soon as the final product is ready to evaluate the impact of the device on persons with dementia and carers before it is released on the market.