While thus far mathematics researchers have tended to concentrate on procedural knowledge, in the last few decades, there has been increasing interest in conceptual knowledge. Therefore, the present dissertation highlights the importance of teaching mathematics conceptually alongside the teaching of procedural knowledge from researchers' and educators' perspectives. In addition, it investigates how teaching for conceptual understanding affects students' achievement in, anxiety about, and attitude toward mathematics.This study draws on interviews with thirty secondary school mathematics teachers from the Erbil city in the Kurdistan Region of Iraq, regarding their views on the conceptual aspect of mathematical knowledge. The three main aspects of the study are focused on: mathematics teacher's perspectives on teaching mathematics conceptually; mathematics teachers' need to teach conceptually, and the obstacles that face them in teaching mathematics conceptually. Furthermore, an experimental approach is utilized to evaluate 200 secondary school students from the same area. In the experimental group, conceptual teaching was the focus. While, in the control group, conventional teaching was used.Pretests and posttests for an achievement test, abbreviated Math Anxiety Scale, and Mathematics Attitude Scale were applied to both the treatment and control groups to reveal the effect of conceptual knowledge on students' achievement in, anxiety about, and attitude toward mathematics, respectively.A thematic analysis of the interviews with secondary school mathematics teachers reveals that they believe that conceptual knowledge is as important as procedural knowledge.They believe that achieving a balance between conceptual and procedural understanding as well as connections between them, are necessary for understanding real mathematics.Furthermore, the pretest and posttest results with secondary school students show that there is a statistically significant difference in mathematics achievement between the two groups (p < .001). Students' attitudes toward mathematics in the treatment group developed positively. Nevertheless, teaching mathematics conceptually reduced anxiety among female students more effective than it did among male ones.