Offices and institutions are the upper-class, aboveground leftovers of urban exploration.

What constitutes an institution?

"Institution" is something of a catch-all term: any large aboveground structure from a mall to a government building to a bank tower risks being lumped into this category.

How much risk is there of getting caught?

As with hotels and hospitals, the odds of getting questioned ("can I help you") are good, but the odds of actually facing any negative consequences are slim unless you do something stupid. In a government building, employees can be expected to have the apathetic "not-my-department" attitude you might expect of civil servants. At night, institutions are usually patrolled by a handful of security guards and janitors.

How much risk is there of getting injured?

In most institutions, there is almost no chance of getting injured in the public areas, and very little chance of getting injured in the off-limits areas.

What supplies should I bring when exploring an office building or institution?

Dress for the occasion. Even if not everyone at the company wears a suit and tie, you're probably less likely to get questioned if you do. Nothing cries "I'm a harmless drone, ignore me" like a well-knotted necktie. Bring along a briefcase and a cell phone. Carry a file folder or a blank sheet of paper around with you until you get the opportunity to trade up to an even better credibility prop, like a company prospectus or something written on a tenant company's letterhead.

When's the best time to infiltrate an office tower?

The absolute optimal time for infiltrating an office tower is between 4pm and 6pm on a Friday night. Between 4pm and 6pm, all the employees are taking off, but the cleaning crews and evening security patrol haven't yet been around. And Friday afternoon is by far the most laid back time at virtually any business. Late nights and weekends are generally a very bad time, involving sign-in procedures, elevators on restricted access, and other hassles.
       If you're going somewhere with high security, or you elect to visit a moderately secure building in the evening, you may have to get around elevators on restricted access (for example, set to only travel to the first three floors of a 50-storey building unless one employs a pass card). One clever way to do this is to simply hop in an unsupervised elevator that goes to the levels you're interested in and wait for someone on one of the higher floors to summon your elevator up. Yes, you lose the ability to plan your route, but dealing with the surprises can be fun.

How do I get inside an off-limits building?

Sneaking past security into off-limits buildings is incredibly easy, once you learn a few tricks of the trade, like avoiding security cameras and motion detectors, taking the stairs instead of the elevators, using the employee walkways. This is amazingly simple in malls, hotels, government institutions, even office towers.

How do I find my way around inside?

"Unlike tunnels, office towers are designed for the general (cattle) public, and hence must be intuitively easy to navigate." -Matthew Landry

How do I get to the off-limits areas?

This really depends on the building, but here are some general tips. If you're after mechanical rooms, look at the building from the outside and count how many levels up to the floors with no windows. It's surprising how often the public stairs and elevators will take you right to these levels.
       If you want to find empty office floors (ridiculously common in Toronto, I don't know about elsewhere), look at the building at night and count how many levels up to the floors with really uncreative lighting patterns. These are floors are usually lots of fun, as they're often still filled with leftovers.
       If you're after the roof, start at the floors two levels down from the roof and find the staircase that goes all the way up by the process of elimination (every building has at least one stairwell that goes to the roof). American fire laws require businesses to label each stairwell either "roof access" or "no roof access", which can save a lot of time and energy. You can probably always trust the "roof access" label, but I have been to several buildings where I was able to access the roof from a stairwell labeled "no roof access" ... though often in a more awkward manner, involving climbing ladders or get climbing around cages.

Where can I see people infiltrating offices and institutions on screen?

Die Hard (1988) - New York Cop John McClane (Bruce Willis) kicks Hans Gruber's (Alan Rickman) terrorist ass all over an LA skyscraper called the Nakatomi building, surviving by hiding in vents and back rooms and the like.

Hard Rain (1998) - After a town gets flooded until abandoned, bad guys chase security guard Tom (Christian Slater) through a flooded school on jetskis.

Hudson Hawk (1991) - Hudson Hawk (Bruce Willis) infiltrates various museums and the Vatican.

Masterminds (1997) - Hacker and troublemaker Ozzie (Vincent Kartheiser) infiltrates his school's ducts and mechanical rooms to harass terrorist Rafe Bentley (Patrick Stewart). Ozzie and friends later chase Rafe through the huge storm drains under the city in dune buggies.

Nick Of Time (1995) - Gene Watson (Johnny Depp) infiltrates an office building to avoid terrorist peer-pressure for him to assassinate the governor of California.

Where can I read more?

Infiltration 1 covers an expedition to the MuchMusic television station, Infiltration 3 features an investigation of the Hudson's Bay Centre, Infiltration 8 offers tours of Metro Hall, Old City Hall and New City Hall, Infiltration 12 looks at Toronto's Union Station.

Where can I look for more information on the web?

Alas, Infiltration seems to be about the only site interested in exploring inhabited buildings.