An introduction to degrees of unsolvability.- The undecidability of the theory of degrees.- Various finite distributive lattices .- Finite distributive lattices .- Linear orderings .- Countable distributive lattices .- Relativizing, a tree of trees, the jump operator.- The homogeneity questions.- Degree theory and analysis.- An introduction to degrees ? 0?.- The undecidability of the theory of degrees ? 0?.