We report the beginning of activity for comet C/2015 ER61 (PANSTARRS), the first instance of watching a longperiod comet turn on. Pre-discovery observations and observations from the NEOWISE space telescope suggest that the nucleus is large, with a radius of R-N similar to 9 km, assuming an albedo of 0.025. Our photometric data follows the comet from r - 8.9 to 4.8 au as it moved into solar conjunction in 2016 July. Our sublimation model shows that activity began near r - 8.8 au (true anomaly, TA = -139 degrees) in early 2015, driven by CO2 sublimation, which peaked in 2016 April at r - 5.1 au (TA = -127 degrees). Appreciable water sublimation began around r - 5.0 au. Our sublimation model is consistent with an active water sublimation area of 1% of the surface (equivalent to 10.2 km(2)), and an active surface area for CO2 sublimation of 0.029% (0.3 km2). The CO2 production rate at r - 4.66 au as measured by NEOWISE is (8.4 +/- 2) x 10(25) s(-1). If CO2-ice had been present on the surface, dust dragged from the surface by sublimation would have been observed much farther out-as far as 20 au. Our thermal models suggest that the CO2 ice was present at a depth of 0.4 m. The comet came out of solar conjunction in 2016 December. and, unless it brightens significantly, is unlikely to have water production rates much higher than a few x10(28) s(-1).