"Dodgy" areas of Tokyo...

[Infection_]

I've heard from many people that Tokyo's an extremely safe city as long as you don't go blatantly looking for trouble, but naturally, there's gotta be some areas of Tokyo that are best avoided, if possible.

What are these? And are there certain areas safe in the day you wouldn't feel to comfortable walking in at night?

[nice gaijin]

I don't think it's so much certain areas, as certain alleyways and side-streets within those areas... Usually one can sense they've made a wrong turn somewhere, it's a good idea to hone that intuition in a foreign country.

I never felt unsafe walking around Tokyo at night; it helps to use common sense.

[Han Chan]

I guess that the area you are looking for must be Kabuki-cho. It's a red-light district which is quite fine during the day but at night it becomes a bit dodgy.

http://en.wikipedia.org/wiki/Kabuki-cho

[GaijinPunch]

I've walked through the darkest of dark alleys in Kabukicho and Roppongi, at very unnatural hours, and have never felt I was even slightly in the midst of danger.

[pipokun]

I bet Okutama area, the western part of Tokyo, is the most dangerous.
When you're lucky, you can see Japanese black bears.
English info
Tsukinowaguma (http://www.asahi-net.or.jp/~xi1k-ymzk/E_HP/English.html)
And there is the highest mountain there, Mt. Kumotori. If you go there without proper preparation in winter, you may also be in danger.

When you realise that you forget buying souvenirs, go to Don Quijote, a famous 24 hour open discount store, at Kabukicho/Roppongi. Roppongi branch close at 7 in the morning. They sell a lot of goods, foods, luxurious watches or even weird school uniforms for your cosplay party.

But some ripping-off bars target tourists as well, so be careful.

[Ewok85]

The only time I've felt threatened in Japan is when I've had police follow me around the neighborhood.... Kabuki-cho and the other "dangerous" areas are perfectly fine really.