Primary health care offers a cost–effective route to achieving universal health coverage (UHC). However, primary health-care systems are weak in many low- and middle-income countries and often fail to provide comprehensive, people-centred, integrated care. We analysed the primary health-care systems in 20 low- and middle-income countries using a semi-grounded approach. Options for strengthening primary health-care systems were identified by thematic content analysis. We found that: (i)despite the growing burden of noncommunicable disease, many low- and middle-income countries lacked funds for preventive services; (ii)community health workers were often under-resourced, poorly supported and lacked training; (iii)out-of-pocket expenditure exceeded 40% of total health expenditure in half the countries studied, which affected equity; and (iv)health insurance schemes were hampered by the fragmentation of public and private systems, underfunding, corruption and poor engagement of informal workers. In 14 countries, the private sector was largely unregulated. Moreover, community engagement in primary health care was weak in countries where services were largely privatized. In some countries, decentralization led to the fragmentation of primary health care. Performance improved when financial incentives were linked to regulation and quality improvement, and community involvement was strong. Policy-making should be supported by adequate resources for primary health-care implementation and government spending on primary health care should be increased by at least 1% of gross domestic product. Devising equity-enhancing financing schemes and improving the accountability of primary health-care management is also needed. Support from primary health-care systems is critical for progress towards UHC in the decade to 2030.