The Islamic astronomical tradition greatly benefited from the many works produced in different ancient cultures, as well as the new empirical and theoretical approaches that led to advancements in various aspects of astronomy. Nevertheless, a significant portion of the history of this tradition, especially its formative period, was lost or remains undiscovered. Avicenna’s addendum to his recension of Ptolemy’s Almagest, which he incorporated into his main philosophical summa, Kitāb al-Shifāʾ, helps recover parts of this otherwise lost history. In the addendum, Avicenna took a historiographical approach and reported the developments in the science of astronomy that occurred from Ptolemy’s time until his own. As such, the treatise also sheds light on the sources that were possibly available to Avicenna. However, because Avicenna did not explicitly indicate his sources, and because the text’s structure is complex and language condensed, analyzing the treatise is a rather challenging task. The present monograph offers a critical edition of Avicenna’s addendum, complete with an English translation and commentary, to assist readers in better understanding and appreciating this valuable contribution to the history of Islamic astronomy.