What determines the size of living organisms? Despite its simplicity, this fundamental question is still largely unanswered. At the microscopic level, the ability of cells to produce organelles of predictable size and shape is fundamental both to the cells dynamic spatial organisation and for the development of multicellular organisms. A particularly apt system to study the regulation of the size of organelles is the eukaryotic cilium or flagellum. Flagella, not to be confused with bacterial flagella, are slender organelles ~10 micron-long (~5 microns for cilia) protruding from eukaryotic cells of almost all species. Poised to be already present in the last common unicellular ancestor of eukaryotes, cilia have been highly conserved throughout billions of years of evolution and are involved in a wide variety of important tasks including motility, sensing (chemical and mechanical), cell-cell communication and mating. Cilia and flagella have an intricate internal structure, called the axoneme, sheathed within a specialised region of the cell membrane. The axoneme combines a stiff passive scaffold based on linked microtubule doublets and thousands of molecular motors arranged in tightly regulated repeat units. This structure grows by polymerisation at the tip, and the necessary material is shuttled there from the cell body by the so-called Intra Flagellar Transport (IFT) machinery. A central question regarding flagella is how do cells manage to regulate their elongation in order to achieve a well-defined length, which they need to perform their tasks effectively. Unfortunately, although a lot is known on IFT, the actual mechanism of tip polymerisation and its regulation to achieve a finite flagellar length are currently poorly understood. The present project tackles this problem with a two-pronged approach combining biophysical experiments and mathematical modelling. We will combine a fluorescence microscopy technique (TIRF) to visualise IFT trains with an optical-tweezer-based technique to monitor nanometre-size fluctuation of the flagellar tip position. This will allow us to link with an unprecedented accuracy the actual polymerisation/depolymerisation at the tip with the traffic of IFT trains. In turn this will provide the necessary experimental data to develop accurate mathematical models of flagellar elongation and length regulation. The results will provide a step change in our understanding of the stochastic growth dynamics of cilia which will critically advance our knowledge of the biophysical mechanism of elongation and size control of cilia and flagella.