A linear algebraic group over an algebraically closed field is reductive (respectively semisimple) if it has no closed, connected, normal, unipotent (resp. solvable) subgroup other than {e}. Important examples are GL_n, SL_n, PGL_n, SO_n, Sp_2n. They are even semisimple (except for GL_n). The goal of the seminar is to continue studying the structure of linear algebraic, reductive, and semisimple groups and to prove the classification statement for reductive groups by root data.