Many different thermal, optical, and thermal/optical carbon analysis methods for organic carbon (OC), elemental carbon (EC) or black carbon (BC) have been applied throughout the world to evaluate visibility and the Earth’s radiation balance. Dozens of intermethod and interlaboratory comparison studies have been conducted. Several of these studies are catalogued and summarized here. BC or EC concentrations are found to differ by up to a factor of 7 among different methods; factor of 2 differences are common. Differences between methods are not consistent among comparison studies, with some methods showing higher BC for one set of samples and lower BC for other sets relative to a common benchmark. The absorption efficiency relating light absorption (babs) to EC that is derived from collocated optical and chemical measurements can vary by a factor of 10, depending on the collocated babs and EC measurement methods. Future intermethod and interlaboratory comparisons must include components that seek to understand the causes of these differences.