A glacier in equilibrium with the climate is characterized by a given geometry. This geometry results from a balance between snow accumulation, ablation (mainly by melting), and the flow that redistributes the mass accumulated at high elevations toward the lower parts of the glacier. If external forcings (climate) or internal forcings (ice flow) change, the geometry of the ice body also evolves. Nowadays, most glaciers around the globe are responding to climate change by a reduction of their thickness. This thinning is the topic of the present article. We will describe how the thinning can be measured, what can cause a glacier to thin, and briefly review the thinning of glacierized areas around the globe.