In the first part of the seminar some basic ingredients of the string models of particle physics are reviewed. The mechanisms to obtain chiral fermion spectrum at the intersection of D-branes or on the D-branes at a singularity are briefly explained. An attempt to construct a model with non-trivial Yukawa couplings is introduced. In the second part of the seminar two systems of D-branes without supersymmetry are introduced; the system of D-branes at non-supersymmetric orbifold fixed point, and the system of so called “brane supersymmetry breaking”. The NS-NS tadpole problem of non-supersymmetric system is discussed. The possible spontaneous gauge symmetry breaking in two concrete models, corresponding to the above two systems, is discussed.