1 Introduction

2 Obstacle problems for plates

3 The classical obstacle problem for the bi-Laplacian

4 The thin obstacle problem for the poly-Laplacian

5 The fractional Laplacian of order s<2

6 The extension problem for the higher order fractional Laplacian

7 The obstacle problem for a higher order fractional Laplacian

8 A two-phase boundary obstacle-type problem for the bi-Laplacian