With the development of mobile devices and Internet-of-Things,the demand for low-power circuits has been growing rapidly. The Analog-to-Digital Converter (ADC) is a key building block in these systems. In this work,we review the progress of low-power ADCs over the years in terms of performance and limitations. From these limitations,it can be shown why sometimes very different approaches are used to minimize power consumption. Various state-of-the-art examples will be shown to illustrate the diversity of techniques and ideas within the field. Next,an outlook to the future is given by discussing various unsolved challenges and new opportunities.