The present status of reactor theory is considered from viewpoints of reactor design and is presented. It is always necessary for reactor designer to compile the newest know-how of calculating technique for reactor analysis in order to reduce design cost. Then, we present a short note for the lastest papers about reactor physics which are appeared in journal and so on, and discuss many unknown factors which should be solved. In Chap. 1, suggestions for reducing the calculation cost are considered. In Chap. 2, many fundamental problems are given; i. e. evaluation of cross section value, solutions of Boltzmann transport equation, neutron slowing down and criticality problems. Review of the lastest calculating methods for criticality calculation is described in Chap. 3. Recent problems about neutron resonance, thermalization and breeding are given in Chaps. 4, 5 and 6, respectively. However, we can not present only short information about time dependent problems because we do not have enough space in our paper to describe them in detail.