The concept of antiaromaticity as applied to 4n-ir electron monocyclic conjugated systems is examined and two sub types distinguished. Relative antiaromaticity is the term which describes systems which are less stable than acyclic con jugated analogues. Absolute antiaromaticity refers to systems which are less stable than even nonconjugated models (e.g., ethylene). The extensive experimental evidence for absolute antiaromaticity (particularly for cyclobutadiene and cyclo- propenide) is critically examined and judged inconclusive in view of the existence of alternate, plausible rationalizations. Theo retical analysis also challenges the reality of absolute antiaromaticity, at least in the monocyclic series. Relative antiaromati city is affirmed, but only for the three smallest monocyclic systems (the two mentioned above plus cyclopentadienylium). Previous to 1965, nonaromatic conjugated systems had sometimes been termed pseudoaromatic. Included in this category, of course, were molecules having monocyclic systems of 4n-7r electrons. In 1965 Breslow concluded that certain 4n monocycles are actually these in cluding at least cyclopropenide (C3-) and 1,3-cyclobutadiene (C4), as well as possibly cyclopentadienylium (Cs+).1 The term antiaromatic denoted and emphasized destabilizing conjugation. Two reference systems were considered for more explicitly defining the conjugation energy. When the reference system is an acyclic conjugated analogue (e.g., 1,3-butadiene for 1,3-cyclobutadiene), the ir electron energy difference represents a relative conjugation energy. Alterna tively, the reference system can be an unconjugated one (e.g., two ethylene ir units for cyclobutadiene), thereby giving an absolute conjugation energy. Based upon an array of experi mental evidence and encouraged by the fact that certain types of MO calculation lead to predictions of negative absolute conjugation energies for C3- and C4, Breslow was impelled to the conclusion that both of these latter are antiaromatic, not only in the relative, but even in the absolute sense. If this con clusion is accepted, a new and basic concept has emerged. The present paper presents a critical evaluation of the key aspects of the proof of absolute antiaromaticity. Experimental Proof Cyclobutadiene. The classic experiment for characterizing the antiaromaticity of a cyclobutadienoid system, devised and executed by the Breslow group, involves measuring the energy change (as £1/2) associated with the transformation of a structure having little cyclobutadienoid character to one having much more and comparing this with the corresponding energy change (£1/2') for an analogous model reaction wherein cy clobutadiene character is absent in both reactant and product.2 The two-electron oxidation 1 «=* 2, e.g., has £1/2 = 0.163 V compared to the E, /2 ' value of -0.113 V for 3 ^ 4. The AE1/2 for 1 <=s 2 relative to 3 <^ 4 is thus 0.27 V (12.4 kcal). This was construed as an approximate quantitative measure of the ab solute antiaromaticity of 2. The approach has obvious merit in that troublesome variables such as angle and torsional strain, hybridization effects, and even substituent effects should be quite small. Fundamentally, however, AE\/2 reflects the dif ference in cyclic conjugation effects between 1 and 2. Breslow's assignment of 12.4 kcal to antiaromatici ty in 2 therefore assumes negligible conjugation effects in 1. Indeed, in principle, it might be at least equally reasonable to assign the cyclobutadienoid conjugation energy of 2 a negli gibly small or even a modest positive value and to attribute 12.4 kcal (or more) to conjugation in 1. Assuming that AE\/2 reflects fairly accurately the difference in conjugation effects between 1 and 2, the basic question of the allocation of the 12.4 kcal as between conjugation in 1 and 2 must be objectively confronted. To analyze the problem further it may be helpful to consider the schematic prototype reaction 5 ^ 6 and to inquire about the magnitude of the conjugation