From glasses and emulsions to colloids and granular matter, amorphous solids show surprising universality in correlations, vibrational properties, and rheology. It has become increasingly clear that constraints of mechanical equilibrium control these universal features. Is there a simple model, analogous to the Debye model for crystals, able to explain this? I will review attempts to answer this question from jamming, large-dimension, and field-theoretic approaches, and argue that the latter is useful and parsimonious. Future challenges and gaps in understanding will be outlined.