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Assemblies of different molecules is a prevalent phenomenon in nature, and crucial in biological life. Most of the biological assemblies are co-assembledcomposites, made up from several different components, either of thesame biomolecular type or a combination of different ones. Knowledge regarding amyloid fibril structure and the mechanism behind it has been elucidated through the development of protocols rendering reproducible kinetic data in buffer systems. The vast majority of these studies have been focused on the self-assembly of the peptides.However, peptide aggregation in vivo typically occurs in a more complex environment, surrounded by either different proteins or lipids, or both.In this thesis, different systems of self- and co-assembly has been studied. We use both top-down and bottom-up approaches to explore the aggregation of the Aβ peptide involved in Alzheimer’s disease. In an in vivo-like environment (cerebrospinal fluid, CSF), we find that Aβ42 fibrillisation is retarded, but occurring through the same mechanism as inferred for Aβ in a buffer system, including the secondary nucleation mechanism. We further investigated the possible effectors of CSF and the minimum component requirement to replicate the effect from this environment through the creation of HDL-like particles. This was achieved through the development of methodology for the expression and purification of ApoA-I from a CSF-free host. Further, the effect of Aβ in the presence of both lipids and other proteins was investigated. We find that we can replicate the effect seen in CSF by the HDL-like particles, in that we see a retarding effect, more pronounced when ApoA-I is added in lipid-free form. We also find that ApoA-I will readily aggregate, and the morphology of the aggregates depend on both extrinsic and intrinsic factors. Finally, we investigated if the inhibiting effect of DNAJB6 on Aβ fibril formation could be reproduced by an isolated chaperone domain. We found that the C-termin