Proper attachment of chromosomes to microtubules is important for the accurate segregation of chromosomes and genome stability. The initial engagement of chromosomes happens along the lateral wall of microtubules through a highly specialised protein structure assembled on the centromeric DNA, the kinetochore. Ultimately, kinetochores must be attached to the ends of microtubules (a geometry called end- on attachment). A series of highly dynamic steps called the end-on conversion process, converts the initial immature lateral attachments into mature end-on attachments. How this process is finely tuned by phosphorylation and dephosphorylation to achieve stable attachments is still unclear. Furthermore, what is the role of microtubule-associated proteins in the stabilisation of kinetochore-microtubule attachments is unknown. This project aimed to study the role of phosphatases in the regulation of the end-on conversion process. First, I investigated the different contribution of the two outer-kinetochore phosphatases - BubR1- recruited PP2A-B56 and KNL1-recruited PP1 - in counteracting Aurora B kinase during the end-on conversion process. I found that BubR1-recruited PP2A-B56 plays an essential role in the process, but KNL1-recruited PP1 does not. I also investigated whether the HEC1/Ndc80 N-tail is a critical substrate of Aurora B phosphorylation for the stabilisation of the end-on attachments. Using a phospho-dead mutant of the HEC1/Ndc80 N-tail, I discovered that cells are still susceptible to Aurora B activity, indicating downstream pathways independent of HEC1/Ndc80. Then, I studied the biological role of the Astrin C-terminus, where an evolutionarily conserved RVMF motif, a putative PP1 binding site, is located. My findings show C-terminal Astrin mutants fail to localise at kinetochores of both monopolar and bipolar spindles; induce defects in the end-on conversion process in monopolar spindles and prolong mitosis time with increased Mad2 levels at the outer-kinetochore. A kinase inhibitor assay showed that kinetochore-microtubule attachment defects in Astrin mutant expressing cells could be rescued when both Aurora B and Cdk1 kinases are inhibited, suggesting a role for Astrin’s C-terminus in counteracting Aurora B and Cdk1 activity. Finally, I probed the putative interaction of the Astrin C-terminus and PP1 using biochemistry, cell biology and fluorescence microscopy techniques. I discovered that artificially targeting PP1 onto the Astrin C-terminus but not on the N-terminus rescues mutants localisation defects at the kinetochore. In summary, my results indicate that Astrin and PP1 interact at the kinetochore of living cells. In conclusion, my work shows that mitotic phosphatases have distinctive contributions in the regulation of the dynamic steps of the end-on conversion process and that Astrin is a potential PP1 phosphatase recruiter at the outer-kinetochore, where is necessary for the stabilisation of end-on attachments.