A long time ago, on this very planet, scientists dreamt of a working fusion reactor.
Fission vs. Fusion: Fission reactors exist in many places. Fission splits heavy atoms (Uranium, Plutonium)
So why is Fusion so elusive? (the combination of light atoms, ie: Hydrogen atoms, to make a Helium). The power of fusion is massive. see any footage of the H-bomb for reference.
Protons and Neutrons are key to fusion. Deuterium and Tritium are the two base combination of a Proton and a Neutron, or a Proton and two Neutrons.
For a little bit more scientific learning, watch the following video about fission and fusion and how they work.
Scientists are now getting to the point where they (deuterium and tritium) are almost close enough to fuse. In 2011, Livermore Labs was expected to reach the "break even" ingnition phase. Despite that, we remain nowhere near having the capability of producing more energy than we're putting in, and certainly not for large markets.
Even though physicists have reached the break even point, they are using all (and more) of the "excess" energy produced, to power the process itself. Energy used to power the lasers needed to fuse the atoms is still massive and more costly than the output energy gain from the process. So essentially there is no real gain in energy yet.
Also, the process of using the energy produced to convert into applied energy, involves heating an element, ie: water. To do this, a "blanket" is put around the water, and a particle that hits it, causes it to heat up. Eventually it gets to a boiling point, and that's what you need to generate higher amounts of usable energy. The problem is that sometimes the particle hits the blanket AOK, and often it hits in different directions and fails to heat the water as desired.
This and many other technical problems are standing in the way of the challenges scientists face.
Is "The Big Lie" (the name given by doubters to the whole idea of practical fusion) on its way to a final defeat? That may well depend on the success or failure of current experiments underway to prove or disprove the potential of better conversion rates of some pellets.
Right now the pellets necessary to do this project are cost prohibitive for any significant scaling up.