The objective of this study is to provide an overall national portrait of trends in elementary and secondary teacher shortages in the U.S. from 1990 to 2021. The primary source of data for this study is the nationally representative National Teacher Principal Survey and its predecessor. the Schools and Staffing Survey, conducted by the US Department of Education. The analyses document that there has been an increase in students and teachers over the past three decades and that a significant number of schools have faced serious difficulties filling their teaching positions. The data also show that the factors behind these staffing problems are complex. There are multiple sources of new teachers. Contrary to conventional theory, only a small part of the new teacher supply is from the pipeline of new graduates from teacher education and preparation programs; far more newly hired teachers come from the reserve pool of former teachers. Combining these multiple sources of new teachers, the data shows that the new supply has more than kept pace with student enrollment and teacher retirement increases, even in the highest shortage fields of math and science. However, when preretirement teacher attrition is factored in, there is a much tighter balance between supply and demand, especially for math and science. Moreover, the data document that teacher turnover varies greatly between different kinds of schools, is especially high in high-poverty schools, and is closely tied to the organizational characteristics and working conditions of schools.