The end or just the beginning for defect-based structure formation

I review the many recent advances in pinning down the predictions of defect-based structure formation scenarios. First, the novel computational aspects of such scenarios are outlined, these include modelling the source and its decay products, taking into account the effects of decoherence and how non-Gaussianity might effect the comparison with observations. These methods are then applied to the standard case where the defects art in a scaling regime throughout the entire history of the universe, except around the time of equal matter-radiation, and the various key observational features of these scenarios are discussed. The most serious problem for defect-based scenarios is the absence of power on very large scales (similar to 100h(-1) Mpc), which appears to be robust to a wide range of possible uncertainties. We then discuss the viability of various alternatives which might be able to produce more power on very large scales.