CONTENTS I. 1. Introduction..................................................................................................................................................................5 2. Preliminaries..............................................................................................................................................................11 3. On Simon’s conjecture..............................................................................................................................................13 II. Pinching theorems for submanifolds of the nearly Kähler 6-sphere..............................................................................16 1. The nearly Kähler structure on S⁶(1)........................................................................................................................16 2. 3-dimensional totally real submanifolds of S⁶............................................................................................................18 3. Totally real surfaces in S⁶..........................................................................................................................................27 III. Surfaces in complex and Sasakian space forms with parallel mean curvature vector...................................................31 1. Totally real surfaces in Kähler manifolds...................................................................................................................31 2. Surfaces of genus 0 with parallel mean curvature vector..........................................................................................34 3. Reduction theorems..................................................................................................................................................51 4. Surfaces of genus 0, C-totally real immersed in Sasakian space forms with parallel mean curvature vector............56 References......................................................................................................................................................................63