This chapter explains the use of psychological theory within a nursing framework to guide them how to assess lifestyle behaviours of patients with Diet and Weight in day-to-day clinical practice. Eating a healthier diet protects from a third of all cancers, diabetes, osteoporosis, heart disease, strokes and tooth decay. Healthy eating is about maintaining a balanced diet. Some assessment are, Waist size: Find the bottom of the ribs and the top of the hips and measure in the middle. The waist-hip ratio is calculated by dividing the waist measurement by the hip measurement. A healthy ratio for men is 1.0 or lower. The ratios above 0.8 for women and 1.0 for men are associated with obesity and are linked to greater risk for health complications and diseases. BMI is a good way of assessing body fat levels for the average person although note difficulties in muscular people and the elderly.