I. INTRODUCTION

II. DATA

A. The DES Y1 Photometric Catalog

B. Cluster Catalog and Associated Systematics

C. Shear Catalog and Associated Systematics

D. Photometric Redshift Catalog and Associated Systematics

III. DATA VECTOR AND ERROR BUDGET

A. Cluster Abundances and Uncertainties

B. Cluster Masses and Uncertainties

C. Systematic Error Budget

IV. THEORETICAL MODEL

A. Model and Data Summary

V. RESULTS

A. Goodness of Fit

B. Cosmological Constraints from DES Y1 Cluster Data

C. Robustness Tests

D. Constraints on the Richness-Mass Relation

VI. DISCUSSION

A. What Drives the Tension Between DES Clusters and Other Probes?

B. What are Possible Solutions?

C. Relation to Other Works

D. Correlated Scatter

VII. SUMMARY AND CONCLUSION