Assessment is one of the key strategies that, if used correctly, can effectively enhance student learning. This study explores senior ESOL (English for Speakers of Other Languages) students' experiences of and attitudes towards formative assessment in the mainstream classroom. The purpose of this study was to investigate how formative assessment might be used effectively to enhance ESOL students' learning from the perspective of senior ESOL students. Data were collected using mixed methods including questionnaires and follow-up interviews with a range of participants from different ethnic backgrounds. One hundred ESOL students participated in the questionnaire and 22 were subsequently interviewed. The questionnaire provided data on the majority ESOL students' experiences and attitudes. Then the interviews allowed participants to describe their experiences and attitudes in more detail. The qualitative methodology used also provided the opportunity for the participants to explain any possible reasons for their attitudes. This study revealed that all the participants had some experiences in some of the formative assessment activities used in classroom. The participants' perspectives also indicated that ESOL students' high expectations for their academic achievement relied on teachers' understanding of their needs as well as effective classroom practice. Feedback was the most favoured formative assessment method by the ESOL students because the students could find out what they had done correctly and where they had gone wrong. Questioning was not liked by the participants, partly because of the language barrier limiting their understanding of the questions, partly because of the way teachers asked the questions (i.e. no wait-time), and partly because of cultural sensitivity (i.e. not wanting to draw attention to oneself). However, the value of questioning as a formative assessment method was recognised by a number of the participants. Self-assessment was liked and found to be useful by some participants. Peer assessment was not liked because of the students' mistrust of their peers' ability to mark their work correctly. Sharing learning objectives and assessment criteria was regarded as an important way to enhance learning as long as teachers provided clear explanations. The study raises questions about the effectiveness of existing formative assessment activities used in classroom and suggests some specific strategies that may help ESOL students learn more effectively. This study clearly indicates that not all formative assessments are equally effective to students of different backgrounds. The choice of formative assessment methods and the way they are administered in class are both important in determining their success for the participants. ESOL students have their own characteristics and needs (e.g. language limitations) and these should to be taken into consideration when choosing and implementing formative assessment methods. The study is of interest in particular to those who teach ESOL students in mainstream classrooms but also has strong links to the field of cross cultural communication, and to the study of effective teaching and learning.