Will my American hair dryer work in Japan?

Only if it is dual-voltage. Most North American hair dryers are 120V and will run very weakly or not at all in Japan. It is better to use the dryer provided by your hotel.

Do I need a voltage converter?

For modern electronics like phones and laptops, no. These are designed to handle 100V-240V. Only use a voltage converter for older, high-wattage appliances, but note that they are heavy and often ineffective.

Are Japanese outlets grounded?

Most wall outlets in Japan are 2-prong and not grounded. Some newer construction or bathrooms may have 3-prong outlets, but do not count on it.

How to handle Japan's power outlets and electrical adapters

Japan uses Type A two-prong, non-polarized outlets, meaning your plugs must have two flat pins of the same width. Most modern phone chargers work, but if you have a grounded plug with three prongs, you will need a three-to-two pin adapter.

Check your plug type. Look at your device plug. If it has two flat, parallel pins, it will fit into most Japanese wall outlets. If your plug has a third round grounding pin, it will not fit without an adapter.
Verify your voltage requirements. Japan operates at 100V. Most electronics (phones, laptops, cameras) are 'dual voltage' (100V-240V) and will work fine. High-wattage devices like hair dryers or straighteners that are not dual-voltage will either not heat up or could potentially blow a fuse if you use a simple converter.
Purchase a non-polarized adapter. If your plug has one pin wider than the other (polarized), it will not fit a Japanese wall socket. Buy a cheap '3-pin to 2-pin' or 'polarized to non-polarized' travel adapter before you leave.
Use a power strip for multiple devices. Japanese hotel rooms often have limited outlets. Bring a small, lightweight power strip from home so you can charge multiple devices using a single wall socket.

Will my American hair dryer work in Japan?: Only if it is dual-voltage. Most North American hair dryers are 120V and will run very weakly or not at all in Japan. It is better to use the dryer provided by your hotel.
Do I need a voltage converter?: For modern electronics like phones and laptops, no. These are designed to handle 100V-240V. Only use a voltage converter for older, high-wattage appliances, but note that they are heavy and often ineffective.
Are Japanese outlets grounded?: Most wall outlets in Japan are 2-prong and not grounded. Some newer construction or bathrooms may have 3-prong outlets, but do not count on it.