Using methods develop in connection with the so called Fermionic Casimir effect [80], a Casimir like interaction energy in fermi systems, we have developed new methods applicable also in the calculation of the Casimir energy in QFT for rather general type of boundaries. Unlike the methods available in literature so far, which are numerically intensive and for which one cannot develop any physical intuition, the approach developed by us is physically transparent, easily generalizable to essentially any geometry and quite easy to implement [81].