Aurelian is usually credited with the idea of the false alarm that got Valerian II to venture out of his tent unarmed in the middle of the night. In 270 AD, he finally got to wear the imperial purple. Within a year, Aurelian had permanently dealt with three other imperial pretenders, and then he got down to the business of reclaiming lost provinces. First, he pushed back the Germanic invaders in the north and set up the first really effective border defensive line in that theater since the one set up by Commodus almost a century earlier. Then he went after Zenobia of Palmyra.
Palmyra was a trading community near the margin of the Roman and Persian empires (in modern eastern Syria). In AD 270-272, Zenobia, who was the dowager of its ruling house and the viceroy for her young son Vaballathus, took control of Roman Egypt, Arabia, and parts of Asia Minor. There is evidence that the Palmyrenes thought Rome would approve the rise of a loyal local power, but Aurelian took the "Palmyrene Revolt" seriously as a threat to his own control. Just pausing to start his great Roman wall-building project, Aurelian moved his legions eastward and campaigned personally against the Palmyrenes. He easily took several cities in Asia-minor and then fought a pitched battle against Zenobia and her army near Antioch. Zenobia's cavalry were easily superior and more heavily armored, but they were tricked into a long chase that exhausted them -- after a while, the lighter Roman cavalry turned on them and prevailed. Aurelian eventually took Palmyra, Zenobia's last stronghold, and she, unlike Cleopatra, chose capture rather than death. According to ancient Roman sources, Aurelian displayed her in his Triumph in Rome in jewels and golden chains, but then he pensioned her off. Some sources say she married a senator and was on her way to becoming a respectable Roman matron when she died a few years later.
Then came those western provinces. The "Gallic Empire" had started in 269 AD when the governor of Gaul, Marcus Cassianius Latinius Postumus, had rebelled against the misrule -- really, the lack of rule -- by Gallienus (while Valerian was a prisoner of Shapur I). Similar usurpations were happening in other areas of the empire as local governors organized local defenses and called themselves emperors. The "Gallic Empire" was simply the largest of these warlord fiefdoms. This situation prevailed until Aurelian reasserted Roman central authority. It is fairly certain that the "reconquest" of the Gallic Empire was really a co-optation deal worked out between Tetricus I, who was the "Gallic Emperor" in 274, and Aurelian. There was only one quick battle to allow Tetricius to save face and then surrender. Tetricus was paraded through the streets of Rome in the usual way at another Aurelian Triumph, but then he was allowed to settle down in southern Italy, where he was given a sinecure job as governor of Lucania. Usurpers in other provinces quickly followed the example of Tetricus and surrendered.
Aurelian is also remembered for his monetary reforms. During the quarter century of chaos that preceded his accession, Roman money had been debased to the point that there was almost no precious metal in the smallish, poorly struck coins. First, Aurelian made better looking coins -- large, well struck, and chemically treated in a silver wash. They at least looked like real money, and that returned some confidence in the imperial currency. A second, more serious reform then put precious metal back into new coins (about 5% silver) and required a value mark on the coins themselves. Coins were still silver-plated, but they also had real value. These reforms were not uniformly applauded, and, in fact, they provoked the only known Roman "mint riot" -- mint workers, undoubtedly bribed and led by corrupt mint officials, fought army troops who eventually restored order. Contemporary accounts set the death toll at 7000. (For a detailed Aurelian monetary history, see a link below.)
But what about those walls? They still run all the way around Aurelian's city right where he put them starting in 270 AD. The project was continued by Tacitus and finished by his successor, Probus. Maxentius finally doubled their height while he was waiting for Constantine's attack, but then he was silly enough to leave fortress Roma and lose a set piece battle at Saxa Rubra in 312 AD. They are pretty much the same as they were in the early fourth century, taking into account minor collapses (like the one in April 2001) and numerous repair jobs. They do not include most of the walls on the Trastevere side of the Tiber, which were added later by the Popes. See links below for much more on the walls.
Aurelian tried to end the round of emperor assassinations by reinforcing the Sol (Sun god) cult and identifying the office of emperor with the god -- something that Nero had tried before him -- but, as in the first instance, it obviously didn't work: Aurelian met the same fate as so many of his predecessors. While on campaign against the Persians, he was assassinated by members of his own military staff after they saw their names on a forged list of persons to be executed: Aurelian's disgruntled secretary had faked the list. (Tacitus executed everyone involved a sort time later.) Aurelian's army held a splendid funeral and raised a great monument for their general, and the Senate quickly deified him.
Aurelian's reign lasted only five years, but his accomplishments were long lasting. He probably added many years to the life of the empire in Rome and in Constantinople.
His life, in much more detail:
De Imperitoribus Romanis (in French)http://www.roman-emperors.org/aurelian.htm
Catholic Encyclopedia: http://www.newadvent.org/cathen/02108b.htm
Monetary reforms and coinage: http://www.geocities.com/Athens/Acropolis/6193/aurelian.html
The Gallic Empire: http://www.roman-empire.net/decline/gallic.html
Zenobia: http://www.roman-emperors.org/zenobia.htm and http://www.ancientroute.com/people/Zenobia.htm
Walls: http://www.mmdtkw.org/VWalls.html and, for more detailed information, http://www.geocities.com/mp_pollett/walls.htm.