The star-formation-rate density of the universe has decreased by a factor of ~ 10 over the last 10 billion years, indicating a major change in either the efficiency with with galaxies form their stars or the availability of star-forming gas over the same time. In this thesis, we study the stellar populations within galaxies during the epoch of peak cosmic star-formation, ~ 10 billion years ago, using infrared observations of the ionized, neutral, and molecular phases of the interstellar medium (ISM) therein. In particular, we focus on two galaxies within this era, both of which have vigorous star-formation as well as active galactic nuclei (AGN), in an effort to learn more about the modes of star-formation at early times as well as the influence of AGN feedback on star-formation. As a first example, we examine the stellar population within 3C 368, a Fanaroff-Riley Class II (FR-II) galaxy at redshift 1.131. Our observations suggest a powerful AGN, accompanied by vigorous and extended star formation. We estimate the age of the starburst and find that it is nearly concurrent with the latest episode of AGN flaring, suggesting a link between the growth of the supermassive black hole and stellar population in this source. Our ionized-gas observations suggest a low gas-phase metallicity within this source, consistent with the weak molecular gas lines that we observe. We suggest that perhaps 3C 368 is accreting pristine molecular gas from an external source, thereby diluting its gas-phase metallicity. For our second example, we investigate the star-formation within SDP.11, a gravitationally-lensed galaxy at redshift 1.783. Our high-resolution gas and dust-continuum observations show intense star-formation in this source extended over several kiloparsecs. Gravitational-lens modeling further indicates that the star formation in this source is extended over a 3-5 kpc rotating disk, suggesting that the starburst present here may not be the result of a major merger, as is the case for local ultraluminous infrared galaxies (ULIRGs), but some other mechanism. The findings presented in this thesis support the idea of a different mode of star-formation being present in high-redshift galaxies. In contrast to local ULIRGs, which have intense star-formation confined to their centers, we find kiloparsec-scale star-forming disks present in galaxies during the epoch of peak cosmic star-formation. While local star-forming galaxies tend to be the product of major-mergers, high-redshift starburst galaxies may well be the product of another process, perhaps accretion of material from the cosmic web.