Dwight and Angela carried on a secret office romance for a long time. The exact time that the relationship began is never specified, but it can be assumed to be in the middle of season 2. They kept their relationship a secret due at least in part to Angela's desire not to be the subject of scrutiny by the rest of the office.
Dwight and Angela went to great lengths to avoid making their relationship public, including avoiding each other at group functions and speaking in code around others. Despite their efforts, however, Pam, Jim and Ryan each became aware of the relationship on separate occassions before it became public knowledge. In Traveling Salesmen, Dwight traveled to New York to deliver files that Angela had forgotten to submit on time. Andy used the incident against Dwight, who quit his job rather than reveal his relationship with Angela. Later, Angela risked revealing the relationship in order to convince Michael to rehire Dwight.
In Fun Run, Angela asked Dwight to care for her cat, Sprinkles. Dwight decided that the cat should be euthanized instead, and it died trying to escape from Angela's freezer after Dwight's unsuccessful attempt to mercy-kill it. This led Angela to break up with Dwight, unable to reconcile his actions with her feelings for Sprinkles. Dwight mourned the breakup greatly, often crying spontaneously and projecting his feelings about the break-up onto his work duties. In Money, Jim discovered Dwight crying in the middle of the night, holding an angel figurine that he had lied about to Angela in order to keep for himself. In a rare moment of sympathy for Dwight, Jim attempted to comfort him back at the office.
In the Season 4 finale Goodbye, Toby, Phyllis discovered that Dwight and Angela had rekindled a racy affair after Angela became engaged to fellow co-worker, Andy. Every time Andy brought up wedding plan information, Angela would secretly page Dwight and meet him in the warehouse for an afternoon delight. Unwilling to share Angela's affections with Andy, Dwight enlisted Phyllis' help, which led to an ultimatum for Angela. Even though her answer was no, he still pursued her until The Duel, when Dwight discovered that Angela had been sleeping with Andy (despite her apparent assurance to the contrary). He then cut off relations with Angela entirely. It was at this point that the (prior) relationship became known by the entire office.
In season six, Dwight and Angela decided to have a baby together, and sign a baby contract. When Dwight later disputed the contract, the two agreed to have sex five times at Angela's request. Angela later broke the contract and began to date The Senator, oblivious to the fact that he was gay. Angela married the Senator between seasons seven and eight and had his baby, though the suspicious circumstances of the birth and the timing of another liaison with Dwight created suspicion that Dwight was in fact the father of the newborn Phillip. Dwight devised a plan to get Phillip's DNA tested in the episode Free Family Portrait Studio, but results of the test indicated that he was not the father.
Throughout season nine, Dwight and Angela were tempted to rekindle their romance. After the Senator revealed his true sexuality publicly, Angela left him and lived in poverty, her life becoming a downward spiral of humiliation and misfortune. She finally admitted to Oscar that she still loved Dwight, and was heartbroken to see him dating another woman. In the episode A.A.R.M., Dwight contemplated his own feelings for Angela, and had a life-changing conversation with Jim. Jim told Dwight that he should not deny the true love that he felt for Angela any longer, prompting Dwight to spontaneously propose to her after pulling her car over by the roadside. Angela then revealed that Dwight was in fact Phillip's father.
The series finale features Dwight and Angela's wedding, and they successfully wed with the rest of their coworkers in attendance.