May and Ben Parker took in their nephew Peter after the death of his parents and raised him. Ben immediately took to the role of the boy's father but May was at first reluctant. She still remembered her parents blaming her own birth for the destruction of their marriage, and she was afraid that Peter might signal the end of her own marriage. In time, however, she warmed up to Peter, who unexpectedly strengthened the couple's marriage. After Ben's death, life became a struggle with money problems and Peter did his best to help.
Peter's secret life as Spider-Man endlessly complicated both his and his aunt's lives. Aunt May lived in constant fear for her "frail" (but now unaccountably secretive) nephew who insisted on a job as a freelance photographer. Parker was determined to capture Spider-Man in action, frequently submitting pictures of him to the Daily Bugle; Spider-Man was a vigilante whom Aunt May feared because of her unquestioning acceptance of the Daily Bugle's smearing.
Meanwhile, Peter feared for his aunt's well-being and the fatal shock that could result if she ever learned about his career as Spider-Man. Peter was often torn between dealing with a major crises while his aunt needed nearly constant care. This conflict took on a bizarre turn when Aunt May had his enemy, Doctor Otto Octavius (also known as Doctor Octopus), as an apparent sweetheart; Peter struggled to deal with his enemy's game while not hurting his Aunt.
Aunt May has "died" several times. In her most recent "death", the "Aunt May" who died was discovered to be a "genetically-altered actress" who impersonated her while May was held captive by villains.
May finally learned about her nephew's secret life once and for all. That resulted in a heartfelt discussion in which Aunt and Nephew confessed their darkest secrets and each learned that the other was far stronger than they imagined.
After their house was destroyed, Peter, Aunt May, and Mary Jane Watson moved into Stark Towers (as Spider-Man had recently joined the New Avengers). Aunt May seemed to be developing a romantic relationship with the Avengers' butler, Jarvis. May was quite the strong-willed woman and even the hard-nosed Wolverine found it impossible to argue with her. During the superhuman Civil War, she and Mary Jane convinced Peter to reveal himself in front of a press conference, and May even made Peter a hand-stitched copy of his original costume.
After Peter joined the anti-registration forces, May was forced to leave Stark Towers. Soon, Kingpin, one of Spider-Man's frequent enemies, ordered a "hit" on the now public-identity Peter Parker. At the motel the Parker's were staying in, a sniper kept watch to eliminate Peter. However, when the sniper fired, Peter dodged, grabbing MJ out of the way. Unfortunately, Aunt May was hit by the bullet -- the bullet intended for Peter.
At the hospital she lapsed into a coma, with death likely. May received a radioactive blood transfusion from Peter, which he hoped would once again save her life due to his mutated healing factor. Although the healing factor was in his blood, the radioactivity did nothing to her system. Peter believed she had built an immunity to his blood, considering this wasn't the first time he had given blood to her.
The demon Mephisto offered to restore Aunt May's health at great personal cost to Peter; his life, and his marriage to his wife, Mary Jane. Agreeing to the terms, Mary Jane and Peter had reality altered so that May could live. Spider-Man's identity was once again a secret from her. The cost of Mephisto's offer was the erasure of Peter and Mary Jane's marriage from all memories.
Since the altering of Peter Parker's life, May was doing volunteer work for a homeless shelter, ran by the seemingly benevolent entrepreneur, Martin Li, who was in fact the super-villain crime boss, Mr. Negative. During her work at the shelter, she met John Jonah Jameson Sr, the father of J. Jonah Jameson, and started a relationship with him. Peter Parker later discovered this relationship when he caught the two of them in bed.
A curious note is that Mephisto's magic that alternate Spider-Man's life in Brand New Day appears to have altered Spider-Man's history with the New Avengers, and has shattered the connection between Jarvis and May. Even more of interest is that later events reveal Jarvis was some time in the past replaced by a Skrull (and a later confirmation it was indeed the Jarvis Skrull who had feelings for May), coupled with Mephisto's magic altering history, complicates matters. Furthermore, in a What If alternate reality, without Mephisto's intervention, May claimed Jarvis was "shot," implying that she saw him die before her eyes, evidently not reverting back to Skrull form, which is even more confusing.