The variation of solar energy entering Earth’s atmosphere, the solar irradiance, is an important influence on the Earth’s climate. Total output of the Sun varies by 0.1% over an 11-year solar cycle and on longer, secular scales there still remains uncertainty. Global temperatures on Earth have increased over the last 160 years along with increasing anthropogenic impact on the environment. It is, therefore, important to fully understand how much of this increase can be attributed to changes in the Sun. This thesis makes use of a semi-empirical version of the Spectral And Total Irradiance REconstruction model, or SATIRE-S, to reconstruct both total and spectral irradiance variations. The physical basis of SATIRE-S is that all variations in solar irradiance are caused by changes in surface magnetic flux emergence. In this thesis, SATIRE-S is updated to accommodate the input of full-disk continuum image and magnetogram data from ground-based and satellite sources spanning three full solar cycles over the period 1974-2009. These changes are described in detail. The combination of terrestrial and space-based images allows for a reconstruction that, for the first time, has been produced independently to, and therefore provides an unbiased comparison of, the composites of direct radiometric observations of total solar irradiance (TSI), which began in 1978. The excellent agreement with, in particular, the PMOD composite supports the simple model assumptions. It also provides constraints on the influence of other mechanisms that may affect solar irradiance on these timescales. The reconstruction of TSI over three solar cycles provides the opportunity to produce a consistent long-term spectral solar irradiance (SSI) dataset that can be put to use by the climate and atmospheric physics communities. This has been one of the goals of this work. In this thesis, comparisons are made with the new SORCE/SIM broadband spectral observations, which include for the first time the visible and infrared above 400 nm. During the declining phase of the recent solar cycle it is found that there is almost no agreement between the model and instrument on a long-term, cycle-length period, but very good agreement on short-term rotational variation. The influence of the ultra-violet region below 400 nm is important in Earth-based stratospheric chemistry and is a strong influence on both temperature and ozone concentration. Therefore, the SATIRE-S spectral dataset is employed in a simple 2D atmospheric model to evaluate its sensitivity in light of recent spectral irradiance changes suggested by data from the SORCE satellite.