Ageing is seen as a progressive loss-of-function over time, generally thought to be due to the intrinsic accumulation of molecular damage. This leads to an increased susceptibility to intrinsic and environmental stressors and ultimately death. Ageing was long thought to be a passive and random process until pioneering studies near the end of the 20th century showed that a single mutation could increase the lifespan of the bacterivorous nematode Caenorhabditis elegans by more than half. This proved that the rate of ageing can be controlled by factors that are encoded in the genome and that ageing is, in other words, a biological process. As biological processes and pathways can be manipulated, this also started the search for compounds that could extend healthy lifespan. Metformin, an anti-glycaemic biguanide drug and the most common treatment of type II diabetes mellitus, has lifespan-extending capabilities both in rodents and nematodes. Several other human diseases, such as cancer and cardiovascular disease are potentially alleviated by metformin treatment as well. This suggests that metformin acts on common pathways involved in a spectrum of ageing-related disorders. Despite its widespread use, its mode of action is largely unknown.Via a quantitative proteomics approach using the model organism C. elegans, we gained molecular understanding of the physiological changes elicited by metformin exposure, including changes in branched-chain amino acid catabolism and cuticle maintenance. We show that metformin extends lifespan through the process of mitohormesis and propose a signalling cascade in which metformin-induced production of reactive oxygen species (ROS) increases overall life expectancy. These results further add to the increasing body of evidence that mild ROS production, while detrimental at higher concentrations, can cause a beneficial adaptive response leading to increased longevity. We further address an important issue in ageing research, wherein so far, the key molecular link that translates the ROS signal into a pro-longevity cue remained elusive. We show that this beneficial signal of the mitohormetic pathway is propagated by the peroxiredoxin PRDX-2. Because of its evolutionary conservation, peroxiredoxin signalling might underlie a general principle of pro-longevity signalling.It was recently shown that neuronal ROS signalling is sufficient to extend lifespan in C. elegans, implying that the longevity-promoting effects of mitohormesis may be propagated by the endocrine system. While the endocrine factors involved are either largely or completely unknown, they are most likely specific neuropeptides. The development of a high-throughput quantitative peptidomics technique for the profiling of these peptides may lead to the identification of these promising mediators of longevity. In this thesis, we described a promising label-based approach for differential peptidomics that may, in time, be used to study the endocrine regulationof ageing. While this work mainly accentuates thepower of differential proteomics, using a more integrated approach combining proteomics, peptidomics, redox proteomics and others may lead to a more comprehensive map of the regulation of C. elegans longevity.