Students in the elementary and secondary grades may use mathematics writing to express ideas and reasoning about mathematics. To understand current classroom practices related to the assessment and instruction of mathematics writing, we conducted a survey of kindergarten through Grade 12 educators across the United States who taught mathematics, with 324 respondents from 44 states. Educators expressed the primary purposes of mathematics writing included helping students learn mathematics and informing mathematics instruction. The majority of educators believed mathematics writing was important yet fewer than half regularly engaged students in instructional activities about mathematics writing. Similarly, only half of educators assessed the mathematics-writing performance of their students. A regression model used to predict the frequency of mathematics-writing activities in classrooms indicated that educators included mathematics-writing activities more frequently in their classrooms if they had higher self-efficacy for teaching mathematics writing, as well as whether they actually provided instruction in mathematics writing or assessed their students in mathematics writing.