Citizens contributing to science started even before scientist became a profession. Traditionally, citizens mainly contribute to data collection through their observations. With the development of sensor technology, and information and communication technology (ICT), citizens are increasingly able to use low-cost sensors to collect environmental data. The overall objective of this thesis is to assess opportunities for informal sensing data and their integration with formal sensing data to improve environmental information provision by investigating challenges and obstacles regarding technical and societal aspects. According to the findings in this thesis, citizens can use low-cost sensors to help them gain information about environmental issues when sensors are calibrated and used properly even though this is still challenging. Citizens and experts do have different perceptions on both formal and informal data. It indicates that application of informal environmental sensing does not only encounter technical but also societal issues. Positively, both citizens and experts consider that informal environmental sensing data can complement formal ones and integrating them can improve environmental data provision in aspects such as spatial-temporal resolution. A conceptual framework for integration of formal and informal environmental sensing data for different environmental purposes is proposed. Five approaches are proposed for integration. A literature study and expert validation was used to evaluate the five approaches based on success indicators. Based on the thesis, advanced air quality sensor calibration and data analysis, integration methods for heterogeneous multiple data sources, and the developments of citizen science and open science and their impact on society are recommended for further research.