This thesis aimed to investigate programmed cell death processes and their role in invertebrate host defence, using the shore crab, Carcinus maenas as the main experimental model. Effort was focussed on two: namely apoptosis and ETosis; the latter being the controlled release of chromatin from the nucleus. Comparison of methods showed that for apoptosis, flow cytometry with bovine lactadherin-FITC and propidium iodide staining was the most effective for quantification in vitro for both mixed and separated haemocytes. By this method, different patterns of cell death were observed in different haemocyte populations and under different life conditions. Additionally, ETosis was demonstrated in two haemocyte types, specifically the hyaline cells and semi-granular cells. The process was found not only to entrap bacteria but also to aid defence by providing the scaffold upon which intact haemocytes assemble during encapsulation; a major cellular process that sequesters non-self particles from the circulation. Importantly, defence cells from the mussel, Mytilus edulis, and the sea anemone, Actinia equina, were also found to exhibit ETotic-like behaviour. As sea anemones lack a dedicated coelomic system, ETosis must predate the evolution of the coelom, showing that it has an evolutionary ancient origin.