Guidance counselors are a common school resource for students navigating complicated and consequential education choices. I provide the first causal estimates of individual coun- selors’ effects on high schoolers, using quasi-random counselor assignment policies in Mas- sachusetts. I find that counselors vary substantially in their effectiveness at increasing stu- dents’ high school graduation rates and college attendance, selectivity and persistence. Coun- selors’ effects on educational attainment are similar in magnitude to teachers’ effects, but they flow through improved information and direct assistance, rather than through improved cog- nitive or non-cognitive skills. Counselor effectiveness is most important for low-achieving and low-income students, perhaps because these students are most likely to lack other sources of information and assistance. Good counselors tend to improve all measures of educational attainment but some specialize in improving high school behavior while others specialize in increasing selective college attendance. Improving access to effective counseling may be a promising way to increase educational attainment and close socioeconomic gaps in education.