長期以來，MM理論就一直接受財務與經濟學者們的挑戰與質疑。這類的質疑主要 可歸納成四種理論：靜態消長(static tradeoff)、資訊不對稱(asymmetric information) 、代理成本 (agency costs)、及市場交互影響 (market interactions)。 靜態消長宣稱最 適資本結構取決於債務的破產成本與節稅間的平衡。資訊不對稱則言股票持有之企業主因較 外界擁有更充分的公司訊息，使發行債券或發行股票隱含公司在經營上不同的績效。代理成 本主張若以股票融資，會使業主增加將公司資源飽入私囊及降低其為公司爭取利潤的動機； 債券融資則可能造成業主投資策略的改變，從保守的投資策略轉為積極的投資策略。市場交 互影響指出若產品價格中反應該公司倒閉機率時，就有所謂的最適資本結構存在。上述的四 種理論在資本結構上引申的涵義亦在本文中加以討論。四種理論衍生出的可觀察變數，提供 實證分析時，用以檢定理論與市場上實際運作的異同。

　　　　　MM theory has been long-time challenged. These challenges come from four theories: static tradeoff, asymmetric information, agency costs, and market interactions. Each is reviewed in this study. Implications to empirical studies are also summarized, and results of existing strudies are tabulated, and compared with the aforementioned theories. This study is organized as follows. Challenges against the MM's theory are presented in the second section. In the third section, factors breaking the MM's environment are classified into four categories. Implications of these factors classified in the third section are discussed in the fourth section. Concluding remarks are proposed in the final section.