During two decades of observing and leading middle school choral rehearsals, the author has observed that teachers frequently repeat exactly the warm-ups they present to their young adolescent choirs at every rehearsal. If teachers know one thing about middle school students, they know that they are constantly changing--physically, intellectually, and emotionally. The vocal warm-up processes used in choirs with changing adolescent voices must, in some ways, be different from the warm-up processes used in choirs of either early elementary children or older high school students. The author examines principles that need to underlie the development of warm-ups that meet the needs of changing voices, encourage reluctant singers, and build toward greater levels of choral success.