We discuss the first detection of anomalous microwave emission, in the Owens Valley RING5M experiment, and its interpretation in the context of the ground-based cosmic microwave background (CMB) experiments of the early 1990s. The RING5M experiment was one of the first attempts to constrain the anisotropy power on sub-horizon scales, by observing a set of -size fields around the North Celestial Pole (NCP). Fields were selected close to the NCP to allow continuous integration from the Owens Valley site. The experiment detected significant emission at both 14.5？GHz and 30？GHz, consistent with a mixture of CMB and a flat-spectrum foreground component, which we termed anomalous, as it could be explained neither by thermal dust emission, nor by standard models for synchrotron or free-free emission. A significant spatial correlation was found between the extracted foreground component and structure in the IRAS 100？μm maps. While microwave emission from spinning dust may be the most natural explanation for this correlation, spinning dust is unlikely to account for all of the anomalous emission seen in the RING5M data. 1. Introduction From the perspective of the 21st century cosmology, it can be hard to imagine how primitive the state of our knowledge was a short twenty years ago and how rapidly the landscape was changing at the time. Today, ground-based experiments like the South Pole Telescope (SPT) and the Atacama Cosmology Telescope (ACT) have measured the high- power spectrum with enough resolution to detect the first nine Doppler peaks (SPT [1, 2], ACT [3]) and enough sensitivity to detect the background of SZ power from unresolved galaxy clusters [4]. The combination of ground, balloon-borne, and space-based missions have already determined fundamental cosmological parameters to uncertainties of a few percent (c.f. DASI [5], ACBAR [6], Boomerang [7], WMAP [8]), and new data from Planck are poised to refine these further. The E-mode polarization of the CMB, whose detection was unthinkable twenty years ago, is now routinely measured by ground-based experiments (first detected by DASI [9, 10], with progressive improvements in resolution and sensitivity by CBI [11], QUaD [12], BICEPI [13], and QUIET [14]), while ever more sensitive limits on the B-mode power spectrum are beginning to place interesting constraints on the tensor-to-scalar ratio [13] (with the next generation cameras like SPTpol, BICEPII, the Keck Array, PolarBear, and ACTpol already in operation). By contrast, the early 1990s had just witnessed the first ever detection of CMB anisotropy on
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