Do you apply flux before or after soldering?

Excuse me, I have a question regarding a technical aspect of soldering. Could you please clarify for me whether the application of flux should take place before or after the soldering process? I've heard conflicting opinions on this matter and I'm trying to ensure that I follow the correct procedure to achieve optimal results. Could you elaborate on the reasons behind your recommendation, and also highlight any potential issues that could arise if the order is reversed? Thank you in advance for your clarification.