If you have firewalls, the physical kind, or severe blockages due to construction, insulation, etc.... Netgear has a great power line based extender as Jeppo mentioned above.
Basically, you plug in a master extender near your main wi-fi router into an electrical outlet. You run an Ethernet cable from the master extender to your main wi-fi router. Then, you plug one or more satellite extenders in areas where you have no signals or bad signals.
Install the software into a laptop or tablet, follow the instructions to set up the network, and you're done.
What you're doing is running your internet signal from the main router, to your Netgear extenders through your electrical wires throughout your house.
This system works very good. Occasionally you need to unplug an extender for a few minutes when it loses an IP address, but it doesn't happen often, at least from my experience.
Works great and the speed is good.