Jigokudani Monkey Park is located about an hour outside of Nagano. The small town of Yudanaka is where most tourists start their trip to the park. Yudanaka is a bit off the regular tourist routes but quite accessible by rail. From Tokyo, the journey is about two and half hours to Nagano and another 50 minutes to Yudanaka. From Nagano Station you need to transfer to the private Nagano Dentestsu Railway- (JR passes are not accepted). For train buffs, there are three types of trains to Yudanaka: the local, rapid and panoramic window train. It is worth the wait to take the panoramic window train and queue for the coveted front seats.
Do not expect pristine natural viewing of these animals; however, the valley into the monkey park is beautiful. You can expect a lot people snapping photos and viewing the monkeys, but you can get within inches of them with your camera to take photos.
The Monkey Park is located a short bus ride from Yudanaka station and then about a 30 minute walk into the valley. During the winter, there is a lot of snow in the park so visitors should prepare for these conditions. The monkeys are by no means wild but they were the highlight of our trip to Japan. We spent an hour or so taking pictures and watching the monkeys relaxing in the hot water. After seeing the monkeys, it is highly recommend having a snack at the café near the bus stop / parking area.