We study the structure and formation of the deeply bound pionic states theoretically. These states have attracted much attention since they provide us with valuable information on the behavior of “real” pions in the interior of the nucleus. In this paper, we briefly summarize the history of the research and, as one of the current activities, we show the isotope dependence of deeply bound pionic states in Sn and Pb isotopes. It is shown the isotope effects can be observed from the data of(d,&hthinsp;3He)reactions with accessible energy resolution. The isotope dependence are expected to provide us new information on the pion-nucleus isovector interaction and/or the neutron density distributions of nuclei. ©2001 American Institute of Physics.