Use this lesson to introduce your students to the similarity of circles. This lesson includes both teaching suggestions as well as a handout for your students to complete as you guide them through the lesson. The lesson (and handout) begin with a brief review of translations, reflections, rotations, and dilations. Then students are guided through several examples in which specific pairs of circles are shown similar. These concrete examples are a springboard for a proof in which all circles are shown similar. After showing all circles are similar, the handout concludes with a small set of problems that can be assigned as a follow-up to the lesson. A key is included. This lesson links to four GeoGebra apps that you can use as you guide your students through the lesson.