Project-based learning (PBL) is a promising teaching method for integrated science education that has gained momentum in educational research and curriculum reforms, especially as a method to enhance 21st century skills and connected worldview. How teachers implement PBL greatly affects students' content understanding and development of skills. The purpose of this qualitative study is to highlight active teachers' PBL practices and their perceptions of the advantages and challenges of implementing PBL to better promote the implementation of PBL in teacher education programs and in integrated science education. This study consisted of two parts: (1) a qualitative-led survey and (2) a case study. First, the data for the survey was collected from January to March 2017 through an online reporting form of an international StarT programme. This programme supports the implementation of interdisciplinary and collaborative PBL in science, mathematics and technology education. 244 teachers from early childhood education to upper secondary school participated from 28 countries. Second, 12 PBL units reported by the teachers were chosen for a case study. The teachers exploited PBL practices that were theme- and inquiry-based, collaborative and engaging to students. However, closer inspection revealed variation and defects in the practices particularly in relation to assessment, using reflection and student-centred approach. In addition, teachers reported several challenges relating to the implementation of PBL. The results indicate that teachers see PBL as beneficial but need support with the implementation. Science teachers' pedagogical competence in PBL could be promoted through collaborative learning in which students, teachers and other participants are learning from each other.