Investigating the cellular processes anti-ageing compounds interact with can identify genes and pathways involved in ageing. The macrolide lactone FK506 was identified in a phenotypic screen as extending lifespan in yeast and C. elegans through an unknown mechanism. FK506 also ameliorates neurodegeneration and age-related weight gain in rodents. Here, the mechanism of action of FK506 has been investigated in two experimental systems: C. elegans and 3T3-L1 mouse adipocytes. As the general mechanisms of ageing are well conserved between C. elegans and mammals, C. elegans has been used to understand how FK506 acts at an organismal level. Firstly, the result of the phenotypic screen was confirmed. FK506 treatment induced lifespan extension in C. elegans in the presence of population crowding stress, but not in the absence of crowding. FK506 treatment inhibited neither E. coli OP50 growth nor C. elegans pharyngeal pumping, demonstrating that FK506 did not induce dietary restriction to extend lifespan. FK506 treatment increased C. elegans thrashing and pharynx pumping rates in early adulthood and delayed accumulation of gut bacteria, showing that FK506 extended healthspan. A transcriptome analysis of FK506-treated C. elegans allowed the identification of transcripts whose levels change and potential pathways by which FK506 manifests its effect. To explore this and to identify potential targets of FK506, the cellular functions required for FK506 to extend C. elegans lifespan and healthspan were investigated using RNA-seq, RNAi, genetic mutation and co-treatment with small molecule inhibitors and inducers. Interestingly, FK506 was found to have different mechanisms of action on lifespan and healthspan. The mechanism of FK506 on C. elegans thrashing rate was DAF-16 dependent, did not require population crowding stress, had a partial interaction with FUdR and autophagy, and may involve Ca²⁺ flux. The mechanism of FK506-induced C. elegans lifespan extension overlapped with dietary restriction and was dependent on calcineurin, TOR-independent regulation of autophagy and the presence of population crowding stress. FK506 may modulate body weight by influencing metabolism and/or acting on adipocytes directly. FK506-treated aged 3T3-L1 adipocytes accumulated significantly less lipid, indicating that FK506 acts directly on adipocytes. RNA-seq of FK506-treated adipocytes found that translation-associated RNAs were upregulated whilst RNAs associated with lipid metabolism were downregulated. An ER-localised FK506-binding protein was up regulated in both C. elegans and 3T3-L1 adipocytes, fkb-4 and Fkbp2 respectively. In conclusion, FK506 has been confirmed as a potential anti-ageing treatment, through its ability to extend lifespan and healthspan in C. C. elegans. In addition, FK506 has also been shown to act directly on mouse adipocytes, resulting in a reduction in lipid accumulation. This action could explain how FK506 caused weight loss in obese aged rats, restoring body mass to a healthy adult weight.