Bicyclopropylidene (1) is capable of undergoing cycloadditions in at least six different modes, depending on the nature of the cycloaddend. In analogy to its [2+ 21 cyclodimerization, 1 adds in a [2+ 21 and [2+ 41 fashion to 1, 3-dienes, most probably via diradical intermediates to a large extent. A concerted mode of formation is likely for at least fractions of the [2+ 41 cycloadducts 11, 14, 17, and 45 and the [2+ 21 cycloadducts 22 and ...