Research in protein aggregation and other degradation routes is of major importance in recent years, influencing the quality and consistency of biopharmaceutical drugs used to treat various life-threatening conditions. In this work we study the open soft-matter physics questions behind such processes that affect developability, safety and efficacy of aqueous solutions of therapeutic proteins. The focus lies on processes of irreversible aggregation and reversible self-association, which can be most influenced by the physical properties of the solution. Physico-chemical interactions reflected in colloidal and conformational stability are evaluated with dynamic light scattering (DLS) and induced protein denaturation. Thermal denaturation is measured via differential scanning calorimetry (DSC). Guanidium hydrochloride and urea are used as chemical denaturants and the ensuing unfolding is measured via tryptophan fluorescent response. The physical effect of excipients such as salts and sugars on protein stability is evaluated. Irreversible aggregation is accelerated mostly with elevated temperature and measured with size exclusion chromatography (SEC), resonant mass measurement (RMM) and micro-flow imaging (MFI). A Smoluchowski type system of differential equations, describing the kinetics of binary aggregation on multiple size scales, is numerically solved in parallel, with the model output compared with experimental data, explaining several phenomena specific to protein aggregation. Adhesion of proteins and protein particles in the micrometer size range to surfaces of containers is characterised with atomic force (AFM) and optical microscopy. We identify several different regimes of particle formation, adhesion to surfaces, and accumulation in bulk solution, shown to have a major impact on determination of particle formation propensity based on elevated temperature studies. We evaluate the discrepancies with mean-field Smoluchowski type models in this non-colloidal regime. Reversible self-association resulting in increased solution viscosity is measured with microfluidic rheometry. The factors contributing to viscosity of protein solutions are analysed and evaluated. A novel interaction mechanism contributing to viscosity is proposed. Finally, this work is a contribution towards advanced protein formulation development with special emphasis on development of biopharmaceuticals.