In the last few decades, Islam has often been associated with radicalism. This perception should be re-examined, because Islam actually teaches ‘moderation’, and the majority of Muslims in fact adhere to moderate understanding and practices of Islam. The radical and conversely moderate attitude among Muslims reflects differences _ sometimes conflicting _ in the understanding and practices of Islam. In fact, there are a good number of schools of thought in Islam in terms of theology, jurisprudence (fiqh), and spirituality (Sufism). The biggest division in Islam is between Sunnism and Shi’ism. This chapter discusses differences among Muslims, particularly between Sunnism and Shi’ism. It also discusses Southeast Asian Islam, which has a distinctive nature of moderation.