SINGLE-PHONON CALORIMETRY, including the measurement of heat flow at the 10^-27 joule level, now appears possible through a combination of nano-engineering and thermometry in the milli-kelvin realm. Michael Roukes and his colleagues at Caltech begin by making a tiny GaAs plate 1-3 microns on a side and only 100 nm thick, supported by 4 bridges each about 100 nm wide. On top of this monocrystalline structure are separate sets of GaAs fingers for gently adding heat to the plate and measuring the subsequent flow of heat. With this setup the group has made the first direct thermal conductance measurements on a nanostructure (to appear in Applied Physics Letters, 19 May). Their present efforts are directed towards measuring the extremely small heat capacity (the energy needed to raise the temperature of an object by 1 K) of their sample, estimated to be about 10^-22 J/K at a temperature of 10 mK. Roukes is in the process of switching from currently- used resistance-based thermometry (a method which itself adds heat to samples at low temperatures) to much less-intrusive measurements based on the thermal noise induced in a SQUID detector. Speaking at the March APS meeting in Kansas City, Roukes said that with this scheme he expects to track the movement of single phonons---single pulses of thermal energy-- -in parcels as small as 10^-26 joules. Physics has not yet seen fit to give a name to a unit as tiny as this. In this single-phonon regime, Roukes points out, many analogies exist between the thermal transport of phonons and the quantum optics of photons.