There are many important parts of a lesson plan, but the absolute minimum segments that must be included are:

Objectives Materials Needed
Time Required Steps to Implement the Plan
Assessment of Students

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It's also helpful to consider including some of the following:

Title of Lesson Standards Addressed
Discussion Questions Background Information
Special Vocabulary

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If you want to really create very thorough lesson plans, include the following as well:

Extension Activities Alternatives to this Lesson
How Well Did the Lesson Work? Entry Level Skills and Abilities
References and Outside Materials Helpful Comments to Implement the Lesson

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Remember, the time you spend creating a good, readable, workable, comprehensive lesson plan will pay off in the long run. Not only will you be able to use it again and again in the future, but other teachers will be able to pick up your plan and use it as well. There are literally thousands of lesson plans to be found on the Internet, as well as a few sample lesson plans to get you started.