Plasticisers are used in the manufacture of polymers and increase the spacing be-tween chains of crystalline polymers to make them more flexible and durable. Phthalates are one of many such plasticisers used in a wide range of consumer goods. Phthalates are not chemically bound to the polymer matrix of plastics and easily leach into the environment and can be absorbed into the human body through the skin, inhalation, or ingestion. Growing evidence suggests that phthalates are environmental toxins and they have been found to be linked to the growth of breast cancer and non-cancerous cell lines in-vitro or involved in inva-sion and metastasis of primary tumours in breast tissue. The mechanism of this is thought to be via phthalates binding to the oestrogen receptor. The work presented in this thesis explores the mechanism(s) through which differ-ent phthalate chemicals promote the progression of breast cancers by exploring phthalates effects on in-vitro breast cancer cells, MCF-7 (oestrogen receptor posi-tive (ER+)) and MDA-MB-231 (oestrogen negative (ER-)) cell line.Presto Blue cell proliferation assays on the effects of phthalates Dibutyl Phthalate (DBP), Diethyl Phthalate (DEP), and phthalate metabolites, Monobutyl Phthalate MBP and Diisocytyl Phthalate (DiCP) on MCF-7 and MDA-MB-231 cells over 72 hours showed subtle and long-term proliferative effects of phthalates on MCF-7 cells and an inhibition of cell proliferation in MDA-MB-231 cells. Clonogenic assays performed over 10 days showed a significant increase in colony number of MCF-7 cells after treatment with 10M DBP and DiCP. Cell proliferation and clonogenic assays suggest phthalates have proliferative effect on ER+ cancer cell line MCF-7. These results suggest phthalates could have clinical implications in the progres-sion of cancer via long term proliferative effects on ER+ cancer cells. The possible mechanisms of phthalates effect on cells was also explored by inves-tigating intracellular signalling pathways of MCF-7 and MDA-MB-231 using western blotting and exploring ER and ER expression in MCF-7 cells using qPCR after treatment with phthalates. The results suggested increased expression of ER after treatment with DBP. Western blotting results showed increased levels of phospho-ERK and decreased levels of phospho-Akt after 72 hours compared to the control treated cells for MCF-7 cells and decreased levels of phospho-ERK in MDA-MB-231 cells after 72 hours of phthalate treatment. Results from confocal microscopy showed a subtle increase in oestrogen receptor expression in MCF-7 cells. These results correlate with the cell proliferation results that showed a proliferative effect of phthalates on ER+ MCF-7 cells and a decrease in cell proliferation compared to the control of MDA-MB-231 cells. Conclusions from this work would suggest the proliferative effects of phthalates are associated with the presence of the ER pres-ence on cells. The latter part of the project included initiating localisation studies of phthalates within cells. As phthalates do not auto-fluoresce the work in this section involved synthesis of a phthalate that was conjugated to a fluorescent chemical. As phthalates do not have a binding site for any of the commercially available fluores-cent tags an amine group was successfully synthesised onto a phthalic acid and could be used in future phthalate localisation studies.The work in this project therefore concludes that phthalates have a moderate long-term effect on breast cancer cell lines and the ability to tag phthalates as demon-strated in this work has the potential to allow more information on the exact site and mechanism of phthalate action in cells to be investigated.