The Bcl-2 family of proteins strictly regulates the intrinsic pathway of apoptosis. Direct physical interactions between Bcl-2 proteins regulate mitochondrial outerpermeabilisation (MOMP), which occurs in response to various cell stresses andapoptotic stimuli. How changes in Bcl-2 protein activity regulate apoptosiscommitment is still unclear, especially with regard to how they interact with eachother within the context of the mitochondrial membrane. Recent studies haveshown that Bcl-2 proteins exist in a dynamic equilibrium between the mitochondriaand the cytosol. In this thesis, by using FRAP, I have measured changes in Bcl-XLand Mcl-1 dynamics in single cells. Surprisingly, individual cells within a populationshow widely differing Bcl-XL and Mcl-1 dynamics. There is a corelation betweenBcl-XL and Mcl-1 dynamics with BH3-only protein expression. Anti-apoptotic andpro-apoptotic Bcl-2 proteins stabilise each other on the OMM. Together, theseresults indicate that cells constantly fine tune mitochondrial priming and thatanalysing anti-apoptotic Bcl-2 proteins by FRAP allows this to be measured at asingle cell level in real time before MOMP.