Enhancing the quality of early childhood education is currently a central goal for many countries. There is widespread agreement that providing preschool teachers with opportunities for professional development (PD) is one of the key ingredients to achieving such a goal. Little is known, however, about the frequency with which preschool teachers engage in the different types of PD activities and about how teachers themselves perceive the usefulness of these activities. Similarly, there is limited research on how participating in PD relates to teachers' self-efficacy beliefs. The present study addresses these gaps in the literature with data collected in a Southeast Asian country: Singapore. Participants were 97 Singapore preschool teachers. A survey composing of several scales was used to collect the data. We explored the frequency and perceived usefulness of seven formal PD activities and 19 informal PD activities, both collaborative and individual. Teachers' self-efficacy was measured with the 'Teacher Sense of Efficacy Scale' (TSES). Our findings showed that: I) Participants engaged in informal PD (both collaborative and individual) more frequently than in formal PD; 2) There were positive correlations between frequency of participation and teachers' perceived usefulness for both formal and informal PD; and 3) High engagement in collaborative informal PD activities was a strong predictor of teachers' self-efficacy beliefs. These results indicate that work-embedded PD, both collaborative and individual, is highly important to Singapore preschool teachers. In order to enhance the impact and responsiveness of PD, we suggest that formal PD should be integrated as part of the informal PD activities in which teachers regularly engage. Limitations and lines for further research are discussed. (author abstract)