18 Gravitational lensing

18.1 Historical remarks
18.2 The dark matter problem
18.3 Gravitational lensing formalism
18.4 Gravitational macrolensing
18.5 Microlensing in the Local Group

18.5.1 Microlensing expectations
18.5.2 First microlensing results and their interpretation
18.5.3 The second period (1996-2000)
18.5.4 Recent developments

19.1 The cosmic ray puzzles
19.2 Magnetic lensing effects