Scaling up health services to meet Universal Health Coverage and post-2015 agenda goals will require continued investment in in-service training (IST) to build the capability of health workers to provide quality health services competently, safely and efficiently. IST already represents a significant share of investments made by Ministries of Health and development partners in strengthening human resources for health (HRH). Yet, in many instances, IST investments have not focused on the development of national continuing education or continuing professional development systems for health workers. IST is also rarely evaluated, which is essential, since IST programs may not always lead to the desired improvements in HRH performance and health care outcomes. In this context, there is growing demand for more effective, efficient, and sustainable health worker training. This series presents five independent articles that address the question, "How can in-service training for health workers be more effective, efficient and sustainable?" Recognizing the lack of adequate literature on how to improve IST and strengthen training systems, this series seeks to promote the use of evidence and evaluation to inform better practices and to maximize training outcomes, as well as to stimulate further research in this field. The papers describe globally recommended practices to improve IST and strengthen training systems (IST improvement framework - forthcoming); review the evidence on educational techniques, timing, setting and media to inform better IST design and delivery; and offer a guiding framework for outcome-level evaluation of IST. Country assessments of the IST situation and lessons learned from two countries highlight the important issues that need to be addressed to strengthen the IST system and improve training. The development process of the Health Worker IST Improvement Framework (to be launched in late 2013, funded by PEPFAR through USAID) catalyzed the exchange of experiences, resources, activities and research amongst global key stakeholders such as Ministries of Health, training program providers, educational experts, professional associations, donors and technical partners. The five articles published through this series were identified during the process used to develop the Health Worker IST Improvement Framework. This process engaged 114 experts from over 26 countries to develop and reach consensus on a set of good practices and guidance to improve IST and strengthen training systems to support better performance and health care outcomes. Redefinition of the HRH global agenda in the context of Universal Health Coverage and post-2015 goals represents a timely opportunity to take stock of existing practices and to identify strategies by which training can be improved. With the publication of this series, the editors and authors call for greater sharing of experiences and innovative research to better inform the development and implementation of strategies to strengthen training systems that can effectively, efficiently and sustainably support health workers to build and maintain their competencies.