1. to perform, to act (as/against), to appear
[verb]
[op-tre-den, trad op, op-ge-tre-den]

"Optreden" is both a verb and a noun (see 2. below). You will use it in two main contexts: an artistic performance or to take action (against).

Examples:
– "De ME trad hard op tegen de protestanten." 
("The riot police acted forcefully against the protesters." The "ME" in Dutch is the "mobiele eenheid": riot police or "anti-riot squad", literally "mobile/agile unit". Note that a "protestant" in Dutch can also mean a "Protestant".)

– "Tegen dit soort gedrag moet direct worden opgetreden!" 
("We have to take measures against this kind of behaviour immediately!" Lit.: "Against this kind of behaviour measures must be taken immediately.")

– "Wat was ook alweer de eerste film waarin Tom Cruise optrad?" 
("What was that movie again in which Tom Cruise made his first appearance?" Lit.: "…the first movie in which Tom Cruise appeared?")

– "Als je je als artiest wil ontwikkelen, moet je regelmatig optreden." 
("If you want to develop yourself as an artist, you have to perform regularly.")

Related words:
– Handelen: to take action [verb] [handelde, gehandeld].
– Actie ondernemen: to take action [verb] [ondernam actie, actie ondernomen].

Example:
– "Na de zoveelste klacht ondernam de gemeente eindelijk actie."
("After the umpteenth complaint, the city council finally took action.")

2. action, way of acting, performance, show

[noun]
[het op-tre-den, de op-tre-dens]

Examples:
– "Het gewapend optreden tegen de protestanten was disproportioneel." 
("The armed action against the protesters was disproportional.")

– "Het optreden van de danseres werd door de recensenten omschreven als dodelijk saai." 
("The critics described the dancer’s performance as dead boring." Note that "danseres" is a female dancer.)

– "Ik kan mij mijn eerste optreden nog erg goed herinneren." 
("I remember my first performance very clearly.")