-ist

(Greek > Latin: a suffix; one who believes in; one who is engaged in; someone who does something)

psychotherapist
A person, usually a psychiatrist or clinical psychologist, professionally trained and engaged in psychotherapy. Currently, the term is also applied to social workers, nurses, and others whose state licensing practice acts include psychotherapy.
pterylologist
ptochologist (s) (noun), ptochologists (pl)
Those who specialize in the study of people who are extremely poor: Ptochologists are finding an increase in the number of people who have been affected by poverty and are existing in shelters (temporary housing for homeless or displaced persons) or on the streets with few or no possessions.
publicist
Someone who arranges publicity for a person or organization by giving information to reporters and broadcasters and arranging public meetings and special events.
puellaphilist
In psychiatry, one who has a special love for young girls and might include a sexual desire for them.
puericulturist
One who raises and trains children.
pugilist (s) (noun), pugilists (pl)
1. Someone who practices the art of boxing.
2. A boxer, a fighter; figuratively, a vigorous controversialist (a person who likes to disagree with other people and say things that make people angry or think about a subject).
pulicologist
pulmonologist
pygophilist
A person who is especially fond of buttocks.
pyretologist
pyritologist
pyrologist
Someone who studies the effects of heat.
pyromachist
Someone who uses fire in military combat or fighting.
pyromantist (s) (noun), pyromantists (pl)
Someone who has the uncontrollable impulse and practice of setting things on fire.