A medical/dental handpiece, comprising: a housing having a base portion, a central portion, distal portion, and a head at an end of said distal portion; said distal portion defining a first axis; said head having a first axial end and a second axial end arranged along a second axis; said first axis intersecting said second axis; said head further comprising a bottom surface at said second axial end defining an opening; a motor having an output shaft (118, 230); said motor being selectively switchable between an activated mode in which said output shaft rotates or reciprocates and a deactivated mode in which said output shaft neither rotates nor reciprocates; an output shaft gear operatively driven by said motor; a light source; said light source comprising one or more light producing elements selectively switchable between an activated mode in which said light source produces light, and a deactivated mode in which said light source does not produce light; a light guide holder mounted in said head of said handpiece for rotation relative to said head; said light guide holder comprising a body having a first axial end and a second axial end, a gear portion shaped and configured to mesh with said output shaft gear, an axial passage extending from said second axial end toward said first axial end of said body of said light guide holder and being opened at least at said second axial end of said body of said light guide holder, whereby said light guide holder is adapted to removably receive a light guide (100, 100A-I) in said axial passage, said axial passage being axially aligned with said opening in said bottom surface of said head, whereby said light source is configured, such that when said light source is activated light from said light source will be directed axially into and through said passage of said light guide holder from said first end of said light guide holder; said light guide holder and said output shaft gear being positioned in said housing such that said output shaft gear meshes with the gear portion of said light guide holder, such that when said motor is activated, said light guide holder will be driven by said output shaft gear; and a control circuit for said handpiece; said control circuit comprising said motor, said light source, and at least one switch; whereby said handpiece is operable in three modes: only the motor is activated, only the light source is activated, and both the motor and light source are activated. a housing having a base portion, a central portion, distal portion, and a head at an end of said distal portion; said distal portion defining a first axis; said head having a first axial end and a second axial end arranged along a second axis; said first axis intersecting said second axis; said head further comprising a bottom surface at said second axial end defining an opening; a motor having an output shaft (118, 230); said motor being selectively switchable between an activated mode in which said output shaft rotates or reciprocates and a deactivated mode in which said output shaft neither rotates nor reciprocates; an output shaft gear operatively driven by said motor; a light source; said light source comprising one or more light producing elements selectively switchable between an activated mode in which said light source produces light, and a deactivated mode in which said light source does not produce light; a light guide holder mounted in said head of said handpiece for rotation relative to said head; said light guide holder comprising a body having a first axial end and a second axial end, a gear portion shaped and configured to mesh with said output shaft gear, an axial passage extending from said second axial end toward said first axial end of said body of said light guide holder and being opened at least at said second axial end of said body of said light guide holder, whereby said light guide holder is adapted to removably receive a light guide (100, 100A-I) in said axial passage, said axial passage being axially aligned with said opening in said bottom surface of said head, whereby said light source is configured, such that when said light source is activated light from said light source will be directed axially into and through said passage of said light guide holder from said first end of said light guide holder; said light guide holder and said output shaft gear being positioned in said housing such that said output shaft gear meshes with the gear portion of said light guide holder, such that when said motor is activated, said light guide holder will be driven by said output shaft gear; and a control circuit for said handpiece; said control circuit comprising said motor, said light source, and at least one switch; whereby said handpiece is operable in three modes: only the motor is activated, only the light source is activated, and both the motor and light source are activated.