Understanding teachers' conceptions surrounding integrated STEM education is vital to the successful implementation of integrated STEM curricula in K-12 classrooms. Of particular interest is understanding how teachers conceptualize the role of the STEM disciplines within their integrated STEM teaching. Further, despite knowing that content-agnostic characteristics of integrated STEM education are important, little is known about how teachers conceptualize the real-world problems, 21st century skills, and the promotion of STEM careers in their integrated STEM instruction. This study used an exploratory case study design to investigate conceptions of 19 K-12 science teachers after participating in an integrated STEM-focused professional development and implementing integrated STEM lessons into their classrooms. Our findings show that all teacher participants viewed STEM education from an integrative perspective that fosters the development of 21st century skills, using real-world problems to motivate students. Our findings also reveal that teachers have varying ideas related to the STEM disciplines within integrated STEM instruction, which could assist teacher educators in preparing high-quality professional development experiences. Findings related to real-world problems, 21st century skills, and STEM careers provide a window into how to best support teachers to include these characteristics into their teaching more explicitly.