转调，是在音乐进行中，从一种调（稳定的调）转入另一种新调的情况。新调被称为“副调”，副调未巩固前称为“离调”（临时转调），当属和弦（调式的五级和弦）、主和弦（调式的一级和弦）全部出现后，才算已经巩固，此时可称作是“转调”。

转调主要包括：

转调能使乐曲增加变化，丰富乐曲的表现力，对音乐发展和进行起到重要的作用。

In music, modulation is most commonly the act or process of changing from one key (tonic, or tonal center) to another. This may or may not be accompanied by a change in key signature. Modulations articulate or create the structure or form of many pieces, as well as add interest. Treatment of a chord as the tonic for less than a phrase is considered tonicization.