School leadership is second only to teaching among school influences on student success, according to research. So what can a school district do to produce a large and steady supply of top-notch school principals--and support their effective supervision? This Wallace Update describes two related Wallace Foundation initiatives seeking answers to that question. The Principal Pipeline effort, begun in 2011, provides funding to six school districts around the country to help them put in place sturdy principal "pipelines," systems aimed at developing strong principals for schools districtwide. Each district is instituting the four components of such pipelines: (1) Clear standards that detail what principals must know and do; (2) High-quality pre-service training programs that recruit high-potential candidates and provide them with training suitable for district schools; (3) Selective hiring procedures that match well-trained principals with the right schools; and (4) Regular evaluation of principals along with the provision of professional development and other support that helps them build on strengths and overcome weaknesses. The initiative is being studied to see, among other things, what impact the pipelines have on schools, including whether they affect student achievement. Among the early lessons are that assistant principals, as the people most likely to step into the top school leadership slot, need development and support, too, and that data systems tracking principals' training, qualifications and performance in schools can help districts in hiring and, potentially, other areas, such as improving pre-service training. The Principal Supervisor Initiative, launched in 2014 and at work in eight additional districts, seeks to strengthen school principals by ensuring that they are effectively managed. That requires shifting the job of the principal's supervisor so it focuses less on compliance matters and more on supporting school leaders, especially in their efforts to bolster classroom instruction.