 He was serving in the Court of Appeals upon his appointment to the Supreme Court in 2002.An expert in the fields of criminal law and procedural law, Callejo Sr. also taught these subjects in various law schools in Manila, including his alma mater San Beda College.Possibly one of the most prolific opinion-writers in recent Supreme Court history, Callejo retired on April 28, 2007 upon reaching the mandatory retirement age of 70.