This paper presents the results of a study undertaken in Portugal to determine the influence of different factors on students' (n=499) decisions to study or refuse to study in one of the physical sciences or engineering. Some influencing factors are related to what goes on in school and during science lessons, and other factors are related to the status of science and engineering careers. Other factors are dependent on individual students themselves, their aptitudes, abilities, and home backgrounds. Some significantly different influences were detected between male and female students and between future scientists and non-scientists. (Author/WRM)