  Among presolar materials recovered in meteorites, abundant SiC and
Al$_{2}$O$_{3}$ grains of AGB origins were found. They showed records of C, N,
O, $^{26}$Al and s-element isotopic ratios that proved invaluable in
constraining the nucleosynthesis models for AGB stars \cite{zin,gal}. In
particular, when these ratios are measured in SiC grains, they clearly reveal
their prevalent origin in cool AGB circumstellar envelopes and provide
information on both the local physics and the conditions at the nucleosynthesis
site (the H- and He-burning layers deep inside the structure). Among the
properties ascertained for the main part of the SiC data (the so-called {\it
mainstream} ones), we mention a large range of $^{14}$N/$^{15}$N ratios,
extending below the solar value \cite{mar}, and $^{12}$C/$^{13}$C ratios
$\gtrsim$ 30. Other classes of grains, instead, display low carbon isotopic
ratios ($\gtrsim 10$) and a huge dispersion for N isotopes, with cases of large
$^{15}$N excess. In the same grains, isotopes currently feeded by slow neutron
captures reveal the characteristic pattern expected from this process at an
efficiency slightly lower than necessary to explain the solar main s-process
component. Complementary constraints can be found in oxide grains, especially
Al$_{2}$O$_{3}$ crystals. Here, the oxygen isotopes and the content in
$^{26}$Al are of a special importance for clarifying the partial mixing
processes that are known to affect evolved low-mass stars. Successes in
modeling the data, as well as problems in explaining some of the mentioned
isotopic ratios through current nucleosynthesis models are briefly outlined.
