Europe's social agenda for the "active elderly" is based upon a series of programs that provide a flexible infrastructure for their lives so that they are motivated, engaged in lifelong learning, and contributing to society. Economically speaking, Europe must engage in active aging research in order to avoid unsustainable health costs, and ambient assisted living (AAL) systems provide a platform for the elderly to remain living independently. This paper reviews research conducted within the area of AAL, and offers a taxonomy within which such systems may be classified. This classification distinguishes itself from others in that it categorises AAL systems in a top-down fashion, with the most important categories placed immediately to the left. In this paper, each section is explored further, and AAL systems are the focus. Entire AAL systems still cannot be fully evaluated, but their constituent technical parts can be assessed. The activities of daily living (ADLs) component was given further priority due to its potential for system evaluation, based on its ability to recognise ADLs with reasonable accuracy.Information 2018, 9, 182 2 of 24 ensure that future system features include the minimum requirements necessary for AAL research and commercial usage, thus ensuring that the elderly and their families have systems capable of meeting their current and future ADL needs. Previous reviews have taken different approaches, yet all failed to provide a framework within which all AAL systems could be classified according to their primary function [5,6]. Some applied specific guidelines to predominantly single-use systems, compared platform performance, or personalised healthcare systems. The evolution of AAL technology [7] has taken us through three generations, from wearable devices that respond to an emergency, to automatic response home sensors, to a third generation combining less-intrusive monitoring with preventive functionalities. Another review approach of surveying ambient environment services [8] under event-, context-, or device-aware services was categorised in terms of a service-oriented, user-centered, and event-aware monitoring service. This review technique compared various ambient services of response time, success rate, low/high-level failures, power consumption, and service reliability/adaption, yet offered no classification. A comprehensive literature review [9] on platforms and systems [10] provided no taxonomy for researchers, and excluded ambient intelligence (AmI) research that is pivotal within robotics and wearable computing. A recent review paper [11] described AAL as the culmination of the following five technologies: sensors, context awareness, machine learning, and artificial intelligence, announcing the importance of device interoperability, yet offering no template for achieving this goal. AALIANCE [12] proffered a roadmap for AAL system standardisation and a possible future evolutionary structure for systems, yet this aspirational template lacks the necessary structure to accommodate curr...