Radiative forcing is a way to quantify an energy imbalance imposed on the climate system either externally (e.g., solar energy output or volcanic emissions) or by human activities (e.g., deliberate land modification or emissions of greenhouse gases, aerosols, and their precursors). The concept of radiative forcing has been central for guiding climate research and policy over the past two decades. There are several reasons for this. It provides a simple yet fundamental index that allows us to look at how climate change is driven by the energy imbalance of the Earth system. It is successful in predicting change in global mean surface temperature as computed from climate models and it, thus, allows quantitative comparison of the contributions of different agents to climate change. It is easy to compute and is reproducible across models and therefore offers a convenient common metric on which policy research and recommendations can be based.