In this thesis I present the results of my PhD research into the physics of accretion (infall) and ejection (outflow) of matter in compact objects.

Accretion disc winds are formed of hot and ionised material launched by magnetic forces, radiation pressure or thermally from the discs of accretors. They have been discovered in most types of accreting systems including supermassive black holes, classical X-ray binaries and Ultraluminous X-ray sources. Outflows form a ubiquitous and important part of the accretion flow, carrying away a considerable fraction of the originally infalling mass. With velocities as high as 30 per cent of the speed of light, their kinetic energy budget can also be significant and have a strong impact on the accretor surroundings. Ultra-fast outflows from supermassive black holes could contribute to or even drive active galactic nucleus feedback in galaxies.

Accretion disc winds can be observed through Doppler-shifted spectral lines in the X-ray part of the electromagnetic spectrum, the wind physical properties can thus be studied with X-ray spectroscopy. In this work, I present the detection of disc winds and I study their physics in a number of accreting systems. I particularly make use of the high-spectral resolution Reflection Grating Spectrometer onboard the XMM-Newton observatory. For efficient data analysis I develop and use systematic automated routines for search of wind signatures in X-ray spectra.

The first two chapters of this thesis introduce the theory of accretion and describe different accreting systems, as well as the X-ray observatories and data analysis methods used in this study. The third chapter contains a systematic search for disc winds in a sample of Ultraluminous X-ray sources, powered by super-Eddington accretion onto stellar-mass black holes and neutron stars. In the fourth chapter I achieve the first discovery of an ultra-fast wind in a neutron star Ultraluminous X-ray source.

In the following two chapters I present the detection of ultra-fast outflows from two accreting supermassive black holes PG 1448+273 and 1H 0707-495. The outflow energetics show that these winds are more than capable to drive feedback in the accretor host galaxies. Furthermore, both show evidence for a multi-phase wind structure, PG 1448+273 also exhibiting variability in time.

In the seventh chapter, I present the discovery of a disc wind in the unique X-ray binary Hercules X-1 known for a precessing, warped accretion disc. I leverage the warped disc precession to sample the vertical distribution of the disc wind, constraining its launching angle as well as the total mass outflow rate, two crucial quantities which are difficult to measure in other accreting systems.

The final chapter contains the conclusions of the thesis as well as potential future research avenues in this field and the promising upcoming X-ray observatories.