Rumors of da Vinci resorting to grave robbery persist to this day, but the truth is that he was allowed to dissect and study corpses at the Hospital of Santa Maria Nuova in Florence.
Leonardo da Vinci's studies of the human skull in 1489 borrowed three-dimensional drawing techniques from architecture that had never been seen applied to anatomical studies before. A new technical vocabulary for anatomical drawings was created and da Vinci's sketches in plan, section, elevation, and perspective marked a massive progression in how the body was documented.
"And you who say that it would be better to watch an anatomist at work than to see these drawings, you would be right, if it were possible to observe all the things that are demonstrated in such drawings in a single figure, but in which you, with all your cleverness, will not see nor obtain knowledge of more than some few veins. To obtain a true and perfect knowledge, I have dissected more than ten human bodies, destroying all the other members, and removing the very minutest particles of the flesh by which these veins are surrounded, without causing them to bleed, except for the insensible bleeding of the capillary veins; and as one single body would not last so long, since it was necessary to proceed with several bodies by degrees, until I came to an end and had a complete knowledge; this I repeated twice, to learn the differences."
Sources: designboom.com: anatomical maps - the renaissance artists' search for perfection; Leonardo da Vinci's Influence on Renaissance Anatomy by Kenneth D. Keele; and Leonardo da Vinci Master Draftsman, the Metropolitan Museum of Art