There are many ways and different terminology used to compute metrics such as response rates for online panels. In a recent effort, the Advertising Research Foundation is promoting an online quality research council to work on different quality topics; one such topic addresses metrics for online panels. This paper starts by reviewing the different metrics proposed by survey and market research associations such as the European Society for Opinion and Marketing Research – (ESOMAR), the European Federation of Associations of Market Research Organizations (EFAMRO), the Interactive Marketing Research Organization (IMRO), and the International Organization for Standardization (ISO) for online data collection. We ground the discussion on the distinction between probability-based online panels and opt-in or access panels. After describing the data collection stages for a prerecruited probability-based web panel and for an opt-in panel, we propose some measures for response, refusal, attrition, and re-interview rates and the relevant formulas. We use concepts derived from existing AAPOR standards for web survey response rates. Complex design and weighted response rates are also addressed. We conclude with a discussion of which metrics can be used with a specific online panel (probability-based or opt-in) and debate the meaning of the different metrics and the relationship between response rates and survey quality.