Howard Wolowitz's father (or commonly known as Mr. Wolowitz) is the deserted husband of Mrs. Wolowitz and the father-in-law of Bernadette Rostenkowski. He left his wife and his son when Howard was 11, which is mentioned in "The Precious Fragmentation", "The Fish Guts Displacement" and "The Habitation Configuration". His whereabouts are currently unknown.
In "The Precious Fragmentation", Howard says that his mother got him an Alf doll when he was 11 to help him sleep after his dad left. He used to pretend his dad had moved to the planet Melmac and Alf was going to bring him back.
In "The Engagement Reaction", a black and white photo of a couple is seen outside the bathroom of Howard's house while he's talking to his mom who is in the bathroom. It is not known if the couple in the photo are Howard's dad and mom.
In "The Closet Reconfiguration", Sheldon finds a letter written by Mr. Wolowitz in Howard's closet. The letter was sent to Howard on his 18th birthday, but he has never read it and refuses to do so by burning it in the sink. Only Sheldon knew what the letter said. When he told all his friends, except Howard, they devised a plan to each tell him a different story about what was in the letter, with only one story being true. Raj told Howard that the letter was a birthday card, saying "Happy birthday, Howard. I love you." Sheldon told him that it was a map to the treasure of One-Eyed Willie, but then Amy points out it was actually the plot for Goonies. Amy told him that his father was in the auditorium on his high school graduation and he cried because he was so proud of his son. Penny told Howard that his dad had a secret life that eventually caught up to him and the only way to keep his family safe was to leave. Leonard told Howard that his father wanted him to know that family is the most important thing and not to throw it away like he did. Bernadette told Howard that inside the envelope was a picture of his father holding him as a baby, with the back saying "Howard my son, my greatest gift." It's never revealed which story is the truth (Sheldon confirmed that his story was fake because he said that he wanted to change his), although Howard states that he wishes they were all true.