This book contains two introductory papers on important topics of nonl inear physics. The first one, by M. San Miguel et al., refers to the e ffect of noise in nonequilibrium systems. The second, by M.E. Brachet, is a modern introduction to turbulence in fluids. The material can be very useful for short courses and is presented accordingly. The autho rs have made their texts self-contained. The volume also contains a se lection of the invited seminars given at the Sixth International Works hop on Instabilities and Nonequilibrium Structures.