Contribution: This article proposes a new definition of conceptual understanding (CU) specific to engineering. It then measures CU of signals and systems (S&S) in senior undergraduate students and describes how students approach conceptual problems. Background: Previous studies across multiple engineering subjects show students have low CU at the end of courses. However, little is known about CU semesters after a course. Research Questions: What is the CU of S&S concepts among electrical engineering senior students? Methodology: This mixed method study uses quantitative concept inventory data (n=467) and think-aloud interviews (n=12) to measure CU. The results come from two universities. Findings: Seniors' scores on the concept inventory are typical of scores presented at the end of an S&S course. Many struggled with the concept of linearity, made a common error when finding the maximum value in graphical convolution, and had low confidence on relating frequencies in time to a Fourier transform representation, but seniors had relatively high CU of time invariance and filtering.