The study of planets around other stars has entered a science-rich era of characterization,

in which detailed information about individual planets can be inferred

from observations beyond discovery and confirmation, which only yield bulk properties

like mass or radius. Characterization probes more revealing quantities such

as chemical abundances, albedo, and temperature/pressure profiles, allowing us to

address larger questions of planet formation mechanisms, planetary evolution, and,

eventually, presence of biosignature gases. The primary method for characterization

of close-in planets is transit spectroscopy. My dissertation comprises transiting

exoplanet case studies using the Hubble Space Telescopes Wide-Field Camera-3

(HST/WFC3) as a tool of exoplanet characterization in a near-infrared band dominated

by broad water absorption. Much of my efforts went toward a characterization

of the WFC3 systematic effects that must be mitigated to extract the incredibly

small (tens to 200 parts per million) signals. The case study subjects in this dissertation

are CoRoT-2b (in emission), WASP-18b (in transmission and emission),

and HATS-7b (in transmission), along with some partial/preliminary analyses of

HAT-p-3b and HD 149026b (both in transmission). I also present an analysis of

transit timing of WASP-18b with HST and other observatories as another clue to

its evolution as a close-in, extremely massive planet purported to be spiraling in

to its host star. The five planets range from super Neptunes to Super-Jupiter in

size/mass. The observability of such planets – i.e. giants across a continuum of

mass/size in extreme local environments close to their respective host stars, – is a

unique opportunity to probe planet formation and evolution, as well as atmospheric

structures in a high-irradiation environment. This genre of observations reveal insights

into aerosols in the atmosphere; clouds and/or hazes can significantly impact

atmospheric chemistry and observational signatures, and the community must better

understand the phenomenon of aerosols in advance of the next generation of

space observatories, including JWST and WFIRST. In conducting these case studies

as part of larger collaborations and HST observing campaigns, my work aids

in the advancement of exoplanet atmosphere characterization from single, planetby-planet,

case studies, to an understanding of the large, hot, gaseous planets as a

population.