When is it appropriate to wear Business Professional to a job interview?

Quick story: I was a nurse case manager for the WTB at Walter Reed Army Medical Center, and then transitioned to Fort Belvoir, VA. I had a Warrior in Transition who was from West Virginia, and I found out about a Jobs conference in Springfield, VA. Knowing that he needed some help, I asked about his resume, which he indicated that he was good to go. I then had him go buy a suit (informed him that the salesman would help him with the tie, shoes, and socks). Unfortunately, the day came and I was out of the office. He was frantic, and my partner in crime helped him tie his tie (this was the first time he had ever worn a suit). He went to the conference and came back to me and informed me that he was one of only three people who showed up in a suit and tie. By the way, he is now working for $70,000 in the National Institute of Standards and Technology.
BLUF: Want the job? Dress the part.

It is always recommended to dress professionally, most of time you can't go wrong being overly dressed. But it does depend on the industry as well. With that said, you are never wrong to dress nice, dress slacks and button down collared shirt at a minimum. If you are going to any kind of professional job always wear a suit and tie (or for women the appropriate equivalent).

I agree with everyone who's responded (just figured I'd add one more). When you go on the interview they'll be looking at you as a representative of their company. You can't go wrong in a suit and tie.

Dress for the Job you want.

I believe you always dress for the position.

If you are going for a job to drive a dump truck you most likely don't need to be in a suit and tie. If you are doing more of a professional environment office job it depends on the scope of your duties. Low level worker bee, probably slacks and a tucked in button up shirt. Upper management, probably need a tie maybe a sport coat. Executive level positions probably a suit and tie.

I got a job as gov contractor a few years ago, I showed up to the interview with dockers and a clean polo and got the job. Another guy wore a full suit and got it also, so I suppose it depends.

You probably will never go wrong with at least khakis and a polo or a casual button shirt. Probably not a good idea to show up with jeans and a T-shirt.

I worked in both the private and public sectors and did interviews in both sectors. In general if you are interviewing for a salaried position as opposed to an hourly job, I recommend the suit or at least a sport coat and tie. I know the IT community is more informal than the management consulting type work I did, but I don't think interviewers would be put off by seeing you in a suit.

Where you work and what your job is make the difference. If you look at my profile picture, you'll see how I dressed for work every day as a GS-14 in a two-star headquarters. When I was a contractor doing military exercise planning, I often wore business casual because that's what the government civilians I worked with wore. At both the two- and four-star headquarters where I worked, the IT technicians--the folks who would come to your desk to install your new computer, etc.-- wore business casual. Sometimes they even downgraded to jeans and a polo shirt if they had to pull wires or work under the floor panels.

If the hiring manager says to wear business casual, do it. If it's not specified, wear a suit.

You have to know the culture of the company you're applying for.

I grew up being taught that a suit and tie says that your mature enough to value the interviewer's time...and responsible enough to own/correctly wear "professional" attire.

Life experience has taught me this is not always correct.

I once showed up for a management position interview in a suit, only to be asked to go out into the lay-down yard and move several pallets of goods. Without hesitation, I stripped the jacket and tie, hopped on the lift, and moved the materials...I got the job. The question is, did the interviewer come up with the "test" because I showed up in a suit, or because that was the test he had in mind before I got there? I think if I had shown up in what the interviewer wore...jeans, boots and a polo shirt...I may have been asked harder questions, and ultimately not been asked to demonstrate my basic skills...ultimately meaning that wearing my suit was a "good" idea; only because I had those skills in the first place.

At another interview (again, for a management position)...the interviewer showed up, again, in casual working attire...I "lost" the job until I walked outside to smoke and found the interviewer already there. After a few minutes, he offered me the job. He explained later that he initially thought I was too "squeaky clean", until discovering my "vice" led into a more informal conversation of my skill sets and experience.

However, I've been to other interviews where not only wearing a suit...but the "right" suit was as important as knowing the quarterly performance of the department one wished to join. Wearing a charcoal suit with a solid blue tie says, "I'm a conformist"...whereas wearing a blue suit with a "slim" red pattern tie says, "I'm a maverick" to many. If your interviewing with a company like Pratt and Whitney...you want to look the "part"; if you're vying for a position with Virgin Atlantic, you don't want to look like an accountant. Ultimately, if either suit comes from Wal-Mart, you're in trouble. I think if you're at that level though...you already know what is, and what isn't acceptable, long before showing up for the interview.

My "go to" choice is usually khaki slacks (pressed), polished but brown shoes, a white oxford collar shirt (cotton) and a basic, blue wool blazer with "regimental" striped gold/blue tie-all of reasonable quality. If the interviewer is in a suit, I'm not uncomfortably "under-dressed", if they're in casual attire, I can quickly remove the jacket and show that I recognize their preferred tone. If I'm actually invited out for drinks after the interview...I'm ready for either a cocktail bar or the "local spot" with a few modifications. Mostly, it shows versatility and preparedness.

Don't be afraid to ask your point of contact in the prospective company (especially if the POC in a hiring manager) what you should wear to the interview. They can give you great insight and help. This will not be seen as a weakness, but rather a desire to make sure that you present the way the hiring managers want to see.

You only have one time to make a 1st impression. I was a contractor and interviewed for the role full time I could have took the approach that I am good, but I decided to wear a suit to leave an impression that I really wanted the role