Disc Winds Matter : Modelling Accretion and Outflows on All Scales This thesis describes the application of a Monte Carlo radiative transfer code to accretion disc winds in two types of systems spanning 9 orders of magnitude in mass and size. In both cases the results provide important new insights. On small scales the presence of disc winds in accreting white dwarf binary systems has long been inferred from the presence of ultraviolet absorption lines. Here the thesis shows that the same winds can also produce optical emission lines and a recombination continuum. On large scales the thesis constructs a simple model of disc winds in quasars that is capable of explaining both the observed absorption and emission signatures – a crucial advance that supports a disc wind based unification scenario for quasars. Lastly the thesis also includes a theoretical investigation into the equivalent width distribution of the emission lines in quasars which reveals a major challenge to all unification scenarios. Introduction Accretion Disc Winds Monte Carlo Radiative Transfer and Ionization The Impact of Accretion Disc Winds on the Optical Spectra of Cataclysmic Variables Testing Quasar Uniﬁcation: Radiative Transfer in Clumpy Winds Quasar Emission Lines as Probes of Orientation and Uniﬁcation Conclusions and Future Work.