This PhD thesis presents work on the atmospheres of both brown dwarfs and exoplanets from an observers viewpoint. The composition and weather of these worlds are explored starting with M-type brown dwarfs and continuing through the L, T and Y spectral sequence, before entering the planetary regime of hot-Jupiters and super-Earths. The similarities and differences between these objects such as their radii, surface gravities, pressures, temperatures and composition are discussed. This thesis presents new results from an extensive near-infrared monitoring survey of a uniform and unbiased sample of 69 L & T dwarfs spanning the L0 to T8 spectral range. Results show that amongst 14 identified variables, nine of them newly identified, variable brown dwarfs are not concentrated at the L - T transition, nor are they observed in a specific colour, or preferentially in binary systems. The thesis also presents narrow-band photometric measurements of the hot-Jupiter HAT-P-1b and the super-Earth GJ~1214b using the 10.4~m Gran Telescopio Canarias (GTC) and the OSIRIS instrument. Results for HAT-P-1b show a strong presence of potassium in the atmosphere caused by a large scale height, possibly due to higher than anticipated temperatures in the upper atmosphere or the dissociation of molecular hydrogen caused by the UV flux from the host star. Results for GJ 1214b, which constitute the first tunable filter measurements of a super-Earth, find no evidence for the presence of methane showing a featureless transmission spectrum consistent with previous studies.

Simple models for bright, helium producing phases in the lives of massive galaxies are used to investigate the distance out to which they could be seen as individual objects. Roughly speaking, objects radiating at effective temperatures of ..;40,000 o could be detected out to redshifts as large as 8 -+12. Such redshifts correspond to densities at which we might reasonably have expected galaxy condensation to occur, except possibly for the lowest part of the probable range of go-values. Such Objects ought to be bluer than ordinary "nearby" galaxies, and for open cosmological models would be expected to be much more numerous than ordinary galaxies; for closed models the numbers of bright and ordinary galaxies should be comparable. The feasibility of detecting such objects by ground -based measures of their integrated skybrightness in the L and M windows is discussed, and it appears that such a technique would be feasible and superior to direct photographic detection only for relatively low effective temperatures in the 20,000 to 1+0,000 range. The possibility of explaining the lack of general Ljy -c4 absorption in distant WO as due to a high degree of ionization brought about by W radiation from these bright galaxies is investigated. The conclusion is that this mechanism will not usually be adequate -- and when it is adequate, the objects causing the ionization should be detectable -- unless the current mean density of uncondensed gas is very low, of the order of 10 -7 particles /cm3 or less.