The collapse of buildings is not a new phenomenon in Nigeria and other developing countries. Evidence from Nigeria has shown that the occurrence of building collapse is mostly prevalent in the informal sector of the building construction industry in Lagos State. Several studies revealed that building collapse is mainly caused by failure of the structural components of a building. Concrete is a dominant material for building structural components in Lagos State, Nigeria and has often been identified as the major cause in many instances of building collapse. This study evaluates the quality control aspects of concrete used especially with respect to its constituent materials, batching and placement in the informal building construction sector of Lagos State, Nigeria using structured questionnaires administered to building construction practitioners working on construction sites in the study area to gather data for the study. Data was analysed using frequency and percentages. Findings were qualitatively discussed. The study found that while the practitioners were conversant with quality assurance aspects of concrete, issues relating to quality of materials used for concrete, trial mix of concrete however, the challenges associated with outsourcing of concrete batching and placement still pose serious threats to concrete quality within the study area. The study recommends that professionals in the construction industry should be involved in all aspects of building procurement and, at the same time, subscribe to quality control measures before and after concrete casting to improve the quality of building construction.