Unethical behaviour occurs when there is violation of the ethical behaviour in relation to individual accountability, personal responsibility for conducting oneself with integrity and creating and maintaining trust in all organizational relationship. Unethical behaviour therefore, signifies something not generally acceptable. Unethical behaviours of secondary school principals in Nigeria include: Embezzlement of school funds, sexual misconducts with students examination malpractices, absenteeism, involvement in politics, lateness to work, other employment outside the school job, abuse and misuse of power, poor supervision of instructions, conversion of school property to private use and favouritism/racism. Hence, causes of unethical behaviours as revealed by the study are as follows: poor and irregular payment of salaries, greed, poor funding of schools, lack of motivation, poverty, poor staffing, illicit relationship with teachers and lack of visionary leadership style. Strategies for curbing unethical behaviour of principals were also highlighted in the book.