How is AMEDD Captains Career Course (CCC) for the reserve component conducted? Is there anything I should prepare for in advance?

There's an extensive FAQ for RC AMEDD officers on HRC's website*.

Specific to the CCC, it is conducted in two phases.
● Phase 1 is 75 hours of asynchronous online instruction
● Phase two is a 2-week resident course (only offered one time a year).

Since the modernization alignment starting with FY23, all CCCs are focused on four learning areas (the last two are different aspects of Command)
● Army Profession & Leadership - Develop personal and professional knowledge to meet the challenges of an evolving complex, operational environment.
● Branch/Warfighting Function/Staff skills - Develop expertise in branch technical and tactical knowledge and skills as well as developing proficiency to serve on battalion and brigade staffs.
● Warfighting & Command - Develop knowledge of combined arms operations and develop proficiency in attributes and competencies to lead company-size units in both field settings and in garrison environments.

If you go to the course repository page* at the Army University and put in your CCC course number (get it from ATRRS), you can see the complete course layout.

One item of advice - if you do nothing else and want a leg-up before going, learn to love the Military Decision Making Process (MDMP) and developing a Concept of the Operations (CONOPS). There are many, many resources online you can refer to as well as the resources you'll have when you do Phase I.

Good Luck!
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* Reserve AMEDD Officer FAQ - https://www.hrc.army.mil/content/Reserve%20AMEDD%20Officer%20FAQ
* Army University (Captains Career Course Modernization) - https://armyuniversity.edu/Organizations/IDesign/CCCModernization
* Army University course catalog - https://armyuniversity.edu/Registrar/CourseCatalog