For the past two decades, the development of preservice elementary teachers' mathematical knowledge and skills has been central to mathematics education research. Two frameworks that researchers have drawn upon to examine such development are mathematical knowledge for teaching and professional noticing (of children's mathematical thinking). We have identified shared theoretical space between these two frameworks, and we hypothesize that effective professional noticing occurs at the intersection of developed mathematical knowledge for teaching and a high level of responsiveness with respect to the mathematical activities of students.