STEAM will explore how solar coronal plasmas are heated in flares and quiescent active regions by measuring the enhancement of elements with low first ionization potential (FIP) in soft (0.5–10 keV) and hard (5–30 keV) X-rays to distinguish signatures of reconnection-based coronal heating mechanisms. Typically, coronal abundances of low FIP elements (Mg, Si, Fe, Ca) are enhanced by a factor of 4 above chromospheric abundances. Measuring the abundances of low FIP elements for various ions at different temperatures provides insight into the coronal or chromospheric origins of the heated plasma. X-ray emissions, including spectral lines and continuum, provide the most direct signatures of hot coronal plasma. Measuring in both soft and hard X-ray allows STEAM to capture a wider range of emission, extending our sensitivity and thus providing a more complete look at plasma evolution. STEAM’s soft and hard X-ray spectrometers will measure individual incident photons and their energies. Combined, the detectors will cover X-ray emissions from 0.5 to 30 keV, with spectral resolutions of < 0.2 & < 1 keV FWHM in soft and hard X-rays, respectively. STEAM will generate spectra with a cadence of 10 seconds, and will be optimized to observe flares of GOES class C1-X1 and active regions above GOES class A1. STEAM will utilize forward modeling with bremsstrahlung and atomic emission databases to fit physical parameters such as temperature and elemental abundance to observed spectral data. STEAM is a student payload hosted on one of the PUNCH Small Explorer spacecraft with an expected launch in mid-2023 and nominal 2-year mission life. STEAM’s spectral observations of solar flares and quiescent active regions in soft and hard X-rays during the rise phase of solar cycle 25 will aid in measuring physical parameters to help constrain potential coronal heating mechanisms.