"Concentration is focusing the mind on one thing, one goal or one activity to the exclusion of everything else. It is a special activity of the mind and not something that you do at every moment. But it is necessary in order to penetrate both the mysteries and the lies that surround all the great truths of life."
"Concentration is an act of giving yourself to something. If you are holding yourself back in life, afraid to make a commitment to anyone or anything, how can you develop concentration or mental power? You will not be able to mount a sufficient effort mentally or physically to get anything done or to comprehend anything of great importance." 
"You must gather your strength and build your focus. If your time and life are being spent mindlessly in all directions, you will not have the power to do this, for power here represents concentration."
"Developing concentration requires the ability to release your thoughts and to control your thinking."
"You will need it to override and overcome the other influences in your life that are dissuading you and holding you back from doing what you know you must do and from discerning a greater direction that Knowledge within you is attempting to provide for you as you go along."
"Stillness cultivates the mind to receive. In stillness you find that things are known already that you have neglected thus far. From these practice periods your mind will become more refined and have greater depth, greater concentration and a greater focus in all aspects of your life."
"To come into the proximity of Knowledge means that you become more and more like Knowledge itself—more whole, more consistent, more honest, more devoted, more concentrated, more self-disciplined, more compassionate and more self-loving. All of these qualities are developed as you approach that which is the source of these qualities."
Power & Influence
"Any mind that is more resolved than yours can persuade you and cast an influence upon you. There is nothing very mysterious about this. It is simply the result of one mind being more concentrated or focused than another. Minds have relative degrees of influence upon one another, depending upon their concentration and depending upon the kind of influence they exert."
"One of the laws in the mental environment is that concentration is power. This has a corollary in the physical environment where physical force is power. In the mental environment, it is concentration. An individual more concentrated than you will have a greater influence on you than you will have on him or her." 
"The person who thinks only of a few things that draw all of his or her emotional intensity will exert a greater influence on the mental environment than someone who casually believes in something or considers something only periodically."
"Concentration is power, and if you are this concentrated, if you have brought all of your inner resources together, and if you are being guided by Knowledge, which is the greatest power, then you will be a force within the mental environment and will be able to help free others from the oppression of all the conditioning that overlays the mental environment of humanity." 
In the Greater Community
"In the future you will be dealing with intelligences that are far more concentrated than you are and have a much greater perspective, having lived with a much broader viewpoint and experience. A more concentrated mind will exert a greater influence upon you than you will upon it. In order for you to engage successfully with a Greater Community intelligence, not only to comprehend it but to offset its influence, you must have a much greater concentration of mind." 
- Wisdom from the Greater Community II, Chapter 31, Concentration
- The Great Waves of Change, Chapter 5, The Deep Evaluation
- Steps to Knowledge, Step 184
- Steps to Knowledge, Step 18
- Steps to Knowledge, Step 113
- Wisdom from the Greater Community II, Chapter 12, Self-Expression and the Mental Environment
- Relationships and the Mental Environment, Chapter 14