Assembly of Amyloid beta (Abeta) peptides, in particular Abeta-42 is central to the formation of the amyloid plaques associated with neuro-pathologies such as Alzheimer's disease (AD). Molecular assembly of individual Abeta-42 species was observed using a simple fluorescence microscope. From the molecular movements (aka Brownian motion) of the individual peptide assemblies, we calculated a temporal evolution of the hydrodynamic radius (R(H)) of the peptide at physiological temperature and pH. The results clearly show a direct relationship between R(H) of Abeta-42 and incubation period, corresponding to the previously reported peptide's aggregation kinetics. The data correlates highly with in solution-based label-free electrochemical detection of the peptide's aggregation, and Abeta-42 deposited on a solid surface and analysed using atomic force microscopy (AFM). To the best of our knowledge, this is the first analysis and characterisation of Abeta aggregation based on capturing molecular trails of individual assemblies. The technique enables both real-time observation and a semi-quantitative distribution profile of the various stages of Abeta assembly, at microM peptide concentration. Our method is a promising candidate for real-time observation and analysis of the effect of other pathologically-relevant molecules such as metal ions on pathways to Abeta oligomerisation and aggregation. The method is also a promising screening tool for AD therapeutics that target Abeta assembly.