Children surviving some types of cancer have a higher risk of developing learning problems after cancer treatment than do children who have not had cancer or its treatment. Cancer treatment may cause problems with learning, attention, and memory. The purpose of this study is to identify brain changes that may underlie learning problems in cancer survivors and investigate whether methylphenidate (a stimulant medication) may reduce these problems. Subjects who have had treatment for acute lymphoblastic leukemia (ALL) or a brain tumor will be asked to take part in this research study. Siblings of some subjects will also be asked to take part, so that their results can be compared with those of children who have had cancer treatment. We hypothesize that children receiving more aggressive therapy will have lower white matter brain volumes and these volumes will be significantly lower than age-matched siblings. We also hypothesize that children who take methylphenidate will show improvements on teacher and parent report measures of attention and social skills.