Acetylene and ethylene are the smallest molecules that contain an unsaturated carbon carbon bond and can be efficiently utilized in a large variety of cycloaddition reactions. In this review, we summarize the application of these C2 molecular units in cycloaddition chemistry and highlight their amazing synthetic opportunities. 1 Introduction 2 Fundamental Features and Differences of Cycloaddition Reactions Involving Acetylene and Ethylene 3 (2+1) Cycloaddition 4 [2+2] Cycloaddition 5 (3+2) Cycloaddition 6 [4+2] Cycloaddition 7 (2+2+1) Cycloaddition 8 [2+2+2] Cycloaddition 9 The Use of Acetylene and Ethylene Cycloaddition for Deuterium and 13C Labeling 10 Conclusions.