I review some of the main nucleosynthesis processes in Asymptotic Giant Branch (AGB) Stars. These include production of $^{12}$C, $^{19}$F, $^{22}$Ne and $s$-elements in the He-rich layers above the degenerate C-O core, and modifications in the abundances of intermediate-mass elements up to Mg-Al, due to proton captures above the H-burning shell. Emphasis is put on the uncertainties still affecting the yields (especially mixing processes and mass loss rates) and on the possible role of planetary nebulae as a source of information on the evolution and nucleosynthesis of the AGB progenitors