Cryonics is the low-temperature preservation of the deceased in order to grant them a chance at a renewed life when technologies for restoration are developed. One of the big outstanding questions is the degree to which a well-conducted cryopreservation successfully preserves the fine structure of the brain and thus the data of the mind. The fact that cold water drowning victims can live again after an hour or more of brain death tells us that memory is encoded in physical structures, with the current consensus (not undisputed) suggesting it is located in synaptic connections between neurons.

Scanning technologies have been used to show that cryopreservation via vitrification of cryoprotectant-infused tissue does indeed preserve the fine structure of brain cell connections. E.g.:

http://www.brainpreservation.org/content/competitors

So it seems reasonable to move forward with initiatives based on the theory that yes, cryonics is going to preserve the mind. Certainly all too many people are left with no other viable option for a shot at a longer life in the future. There is always the need for more and better proof, however. At the present time studies involving restoration of vitrified individuals must be carried out in lower animals, unfortunately, as researchers are still far from the point at which they can safely restore vitrified mammals. That said, restoration of a vitrified mammalian organs has been demonstrated, though even that is very early stage work in the grand scheme of things.