My current research focuses on protein diffusion on the plasma membrane. Single particle tracking is a powerful technique often used in the study of dynamic mechanisms on the cell surface such as binding, confinement and trafficking. A potential problem in the analysis of single particle trajectories is to account for transitions between modes of mobility. I have developed a new statistical method based on hidden Markov models that detect transient periods of drift diffusion, reduced mobility and spatial domains within single trajectories due to transient associations with other biomolecules. This method can also detect spatial domains containing either high or low concentrations of these associated biomolecules.