To evaluate the colour masking ability of caries infiltration system, this invivo study was carried out on a total of thirty anterior teeth with inactive post-orthodontic white spot lesions on the facial surface, selected from seven children aging from 9-16 years old of the outpatients’ clinic of Orthodontic Department, Faculty of Oral and Dental Medicine, Cairo University. Teeth were free from any enamel defect. They were divided randomly into two groups and each group contained fifteen teeth receiving different treatments.Group 1:Children treated with fluoridation: a commercial fluoride varnish containing fluorosilane (0.1% fluoride), (Fluor Protector®, Ivoclar Vivadent, Liechtenstein).Group 2:Children treated with caries infiltration (ICON® preproduct; DMG, Hamburg, Germany).The colorimetric parameters of each white spot lesion were assessed, using an intraoral spectrophotometer, before, immediately after treatment and at 24 hours, 1 week, 1 month and 6 months later to determine the colour changes.The results of the study revealed that:Resin infiltration improves the colour of white spot lesions with adequate durability up to 6 months.After 1 week, 1 month and 6 months, ICON® group showed higher means of colour change, when compared to baseline, than Fluor Protector® group.Through the whole study period (From immediately after application to 6 months), the ICON® group showed a statistically significant increase in colour change.While in the Fluor Protector® group, there was no statistically significant change in colour at different time intervals.To assess the efficacy of resin infiltration in preventing lesion progression and the degree of its penetration inside the lesion, an invitro study was carried out on a total number of twenty freshly extracted sound human first and second primary molars.Artificial enamel subsurface lesions were created on the proximal surfaces. Then, teeth were equally and randomly allocated into 2 groups:Group 1:To assess the materials’ penetration inside the lesion. The mesial surfaces were resin infiltrated while the distal surfaces were coated with fluoride varnish. Then teeth were sectioned mesiodistally to be examined using SEM.Group 2:To assess the efficacy in preventing further decay progression. Teeth were then equally and randomly allocated into two subgroups:Subgroup a:Included five teeth, the mesial surfaces were resin infiltrated and the distal ones were left untreated (negative control).Subgroup b:Included five teeth, the mesial surfaces were treated with fluoride varnish (positive control) and the distal ones were left untreated (negative control).Then teeth were secondary demineralized and examined using SEM and EDX before (baseline) and after initial demineralization, after materials applications and finally after secondary demineralization to compare Calcium/Phosphorus ratios.The results of the study revealed that:The ICON® material showed a significant amount of infiltration into the porosities of the lesion. While the Fluor Protector® material did not show any infiltration. The varnish material stayed on the top of the lesion and blocked the porosity of the lesion superficially.After a new acid challenge, both of ICON® and Fluor Protector® treatment modalities were equally effective in prevention of further demineralization of the tooth without any statistical significant difference between them.