  In connection with the space-time uncertainty principle which gives a simple
qualitative characterization of non-local or non-commutative nature of
short-distance space-time structure in string theory, author's recent
approaches toward field theories for D-branes are briefly outlined, putting
emphasis on some key ideas lying in the background. The final section of the
present report is devoted partially to a tribute to Yukawa on the occasion of
the centennial of his birth.
