The occurrence and intensity of some natural hazards (e.g. hydro-meteorological) increase due to climate change, with growing exposure and socio-economic vulnerability, leading to mounting risks. In response, Disaster Risk Reduction policy and practice emphasize people-centred Early Warning Systems (EWS). Global policies stress the need for including local knowledge and increasing the literature on integrating local and scientific knowledge for EWS. In this paper, we present a review to understand and outline how local and scientific knowledge integration is framed in EWS, namely: (1) existing integration approaches, (2) where in the EWS integration happens, (3) outcomes, (4) challenges, and (5) enablers. The objective is to critically evaluate integration and highlight critical questions about assumptions, goals, outcomes, and processes. In particular, we unpack the impact of power and knowledges as plural. We find a spectrum of integration between knowledges in EWS, mainly with dichotomy at the start: focus on people or technology. The most popular integration approaches are participatory methods such as ‘GIS mapping’ (technology) and methods that focus on ‘triangulation’ (people). We find that critical analysis of power relations and social interaction is either missed or framed as a challenge within integration processes. Knowledge is often seen as binary, embedded in the concept of ‘integration’. It is important to know what different knowledges can and cannot do in different contexts and acknowledge the hybrid reality of knowledge used for EWS. We argue that how we approach different knowledges in EWS has fundamental implications for the approaches to integration and its meaning. To this end, attention to the social processes, power dynamics, and context is crucial.