*Frontmatter, pg. i*Table of Contents, pg. iii*1. Introduction, pg. 1*2. Preliminaries, pg. 6*3. Motion by mean curvature, pg. 18*4. Existence of varifolds moving by their mean curvature, pg. 33*5. Perpendicularity of mean curvature, pg. 121*6. Regularity, pg. 161*Appendix A: Grain growth in metals, pg. 224*Appendix B: Curves in R2, pg. 229*Appendix C: Curves of constant shape, pg. 235*Appendix D: Density bounds and rectiflability, pg. 238*Figure captions, pg. 240*Figures, pg. 241*References, pg. 251