A teacher (also known as a school teacher, or, in some cases, an educator) is someone who helps others acquire knowledge, ability, or values.

Informally, anyone can play the role of a teacher (for example, when showing a colleague how to perform a specific task). In some countries, it is possible to teach young people of school age in an informal environment (e.g. within the family (family school education)) rather than in a formal environment such as a school or university. Other professions may require a lot of teaching (e.g. young workers, priests).

In most countries, students are usually taught by paid professional teachers. This article focuses on people employed as key players who should teach in a formal education environment (such as in schools or other places where they receive initial formal education or training).

Teachers can provide instruction in literacy and numeracy, craft or vocational training, art, religion, citizenship, community roles or life skills.

Formal teaching tasks include preparing courses, teaching and evaluating students' progress according to the agreed courses.

Teachers' professional responsibilities may be beyond the scope of formal teaching. Outside the classroom, teachers can accompany students on field visits, supervise the study room, help organize school activities, and act as the supervisor of extracurricular activities. In some education systems, teachers may be responsible for students' discipline.

In many countries, the person who wants to be a teacher must first obtain the specified professional qualification or certificate from the University. These professional qualifications may include pedagogical research, teaching science. Like other professionals, teachers may have to or choose to continue their education after they are qualified. This process is called continuous professional development.

The problem of teachers' qualification is related to the position of the major. In some societies, teachers enjoy the same status as doctors, lawyers, engineers and accountants. In other societies, the status of this profession is lower. In the 20th century, many smart women could not find jobs in companies or government departments, so many chose teaching as their default occupation. As women are more and more popular with companies and governments today, it may be more difficult to attract qualified teachers in the future.

Teachers are usually required to take an initial education course at the school of education to ensure that they have the necessary knowledge, ability and compliance with the relevant code of ethics.

There are various institutions designed to inculcate, preserve and update teachers' knowledge and professional status. There are many normal colleges all over the world. They may be controlled by the government or the teaching industry itself.

They are usually established to serve and protect the public interest through certification, governance, quality control and the implementation of practical standards for teaching majors.